SCP Foundation

All testing must be previously approved by the Project Lead, and performed by the volunteer while isolated in a SHA1 containment cell with closed circuit digital recording.

"All testing must have prior approval by the current Project Lead. Testing must be performed with a Class-D subject isolated in a SHA1 containment cell with closed circuit digital recorders running."

Mobile Containment Task Force Gamma-27 ("Wet Blankets") are responsible for the tracking and acquisition of future discovered instances of SCP-XXXX.

"Mobile Task Force Gamma-27 ("Wet Blankets") will be responsible for tracking and acquiring any uncontained instances of SCP-XXXX."

SCP-XXXX refers to a pouch, measuring 8 cm by 16 cm, and a reddish powder contained inside.

Ouch… Just ouch.

"SCP-XXXX is a pouch measuring 8 cm by 16 cm containing an unknown reddish colored powdered substance."

You need more description here. What is the pouch made of? Tanned human skin? Artificial snake leather? The hides of 50,000 midget ginko lizards? What color is the pouch? Does it have a drawstring? A Velcro close? A zipper? What does the powder look like? Is it granules like sugar? Is it really powdery like confectioner's sugar? Does it flow or does it clump?

The pouch is contained within packaging that denotes it as "Professor Pizazz's (sic) Party in a Pouch™".

You jump from describing the pouch and powder to describing the packaging. Keep things together and it reads much better.

The rest of the word torn from the packaging, likely caused by the removal of the price tag by the previous owner before containment.

Really unnecessary detail.

See Addendum XXXX-1 for further information on attempts to contact this e-mail address.

Personal style point, but the addendum should immediately follow the section mentioning it so people don't have to hunt all over your article for it.

The pouch itself is of the same reddish coloring as the powder, and has the phrase "It's Got Pizazz!" (sic) on both the front and back of the pouch. There is a black pull-string as well, allowing for the pouch to be closed tightly to prevent powder from escaping.

This needs to be up further by where you first were talking about the pouch and the powder. Rewrite suggested below.

"The pouch is of the same reddish color as the powder within. Printed on both sides of the outside of the pouch are the words, "It's Got Pizazz!" (sic). The pouch has a black, nylon drawstring around the mouth of the pouch that can be drawn closed to prevent the powder from being spilled."

Both the pouch and the powder contained within possess a strong copper odor.

This needs to be included with the line about the color and texture of the powder.

Once the directions are properly followed the anomalous effects of SCP-XXXX take effect.

"If the directions are followed as printed, the anomalous effects of SCP-XXXX will manifest."

Once opened, the object the powder was applied to will contain one of a variety of different effects.

This makes no sense, it's an incomplete sentence that needs major re-work. I am not even sure how to suggest a re-write for it.

After certain conditions are met the occurrence will end, dissolving into a small pile of reddish powder. The powder can then be swept up and returned to the pouch.

Cop-out to say that certain conditions must be met and then not defining those conditions. That's just being lazy. If you are going to say that conditions must be met, define the conditions.

Initial discovery and recovery of SCP-XXXX took place on 09/15/2013 when Agent ███████ was in attendance at the Bar Mitzvah of a relative.

"Initial discovery and containment of SCP-XXXX occurred on ██/██/████. Agent ███████ was attending the Bar Mitzvah of a relative when…" and re-work from there.

Clad only in a bikini, the female sang "happy birthday" to the room before collapsing into a pile of reddish powder.

"Happy Birthday" is a copyrighted song and thus needs to be capitalized. Use of the title of the song may be covered under copyright law, so you might want to just use H████ B███████.

Agent ███████ recovered the powder, and upon interviewing witnesses recovered SCP-XXXX.

"Agent ███████ recovered the powder, and after interviewing witnesses seized SCP-XXXX from [REDACTED]."

The previous owner claimed to have purchased the item online, but no attempts made to locate any additional instances of SCP-XXXX proved successful. See Addendum XXXX-2 for details.

I would just put: "The previous owner of SCP-XXXX claims to have purchased the item from an online store, but was unable to recall the exact site."
You already stated above that attempts to locate the web site were in Addendum 1, and here you are referring to it as Addendum 2. Which is it?

Once opened, box contained a marching band numbering 20, each member no more than 4 cm tall.

"When opened, the box contained a marching band consisting of 20 humanoids no more than 4 cm tall."

SCP-XXXX applied to small laundry basket, which then is covered completely in a large tarp. Once opened, 99 red balloons floated upwards from the basket, despite the basket not having the volume necessary to hold that many balloons. Dissipation triggered when balloons reached ceiling of test area.

This is hilarious! Might want to include something about a 1980's pop song playing until the balloons dissipated.

(Only need collapsible on the top. Not necessary to put top and bottom. Take this part out: hideLocation="both")

Right before the researcher sent the email, however, they received the following message:

Clinical tone is off. "Prior to the researcher sending the email, the following message was received."

Investigation of server data was unable to locate where the e-mail was received from.

Clinical tone. "Investigation of the IP address and DNS source of the email has so far been unsuccessful as no such IP or DNS location exist."

All further attempts to reach this e-mail address received the same reply immediately before being sent, regardless of the content of the message.

"All subsequent attempts to contact someone through this email address have received the same reply prior to the email actually being sent regardless of the content of the outgoing email message."

This suggests it is an automated response, but research is still ongoing to try to identify how the replies were received before the message was sent.

"This suggests that perhaps the email is an automated response to messages directed to the address. Research is ongoing how the replies are being received prior to the message actually being sent."

Gamma-27 responds to reports of possible anomalous activity in the city park of ███████, Arizona on 05/01/2014.

Clinical tone. "On ██/██/████, MTF Gamma-27 was dispatched to reports of anomalous activity in the city park of ███████, AZ."

Upon arrival at the scene, Gamma-27 initiates contact with a large green humanoid armed with a sword and shield.

"On arrival at the scene, MTF Gamma-27 initiated contact…"
These events occurred in the past, clinical tone is broken when you do not refer to them in the past tense.

ork raid

Standard spelling is "Orc" with a capital O since it is a proper noun.

The whole conversation feels forced and unnatural. I would suggest practicing it in front of a mirror to get a feel for how it should be written.

Class-A amnestics were applied to all involved in the incident and a cover story put into place.

Congrats on using the correct version of "amnestic". However, it needs work. "Class-A amnestics were given to all witnesses and a cover story initiated to explain the incident."

When the store mentioned in the interview was investigated, the building was gutted by a fire.

"When the store was investigated, it was discovered that it had recently been completely gutted by a fire of unknown and suspicious origin. Investigation into the source of the fire is ongoing."

The investigation following this event showed the cause to be arson, with no further leads found at the scene.

Just leave this out.