SCP Foundation

The idea is an entity that can only be perceived by reading the note that refers to it. The effect can only take place under one parameter: the subject who reads the note must have forgotten at least one memory deeply significant to them.

The entity would not consciously know the memory, but tends to hint about the memory when a subject interacts with the entity. Once the affected subject has remembered the memory, they will be unable to perceive the entity and will have no recollection of any experiences with the entity. A memento will materialize in the subject's vicinity that varies based on the experience the subject had with the entity.

As far as the entity itself, it is not affected by its environment to certain extents and has limited ability to interact with objects in the environment. For anyone who can't perceive the entity, they will be unable to perceive any changes in the environment made by the entity.

This is only the base idea, I just wanted to pass this through the brainstorm section

The effect can only take place under one parameter: the subject who reads the note must have forgotten at least one memory deeply significant to them.

How would a researcher be able to figure that out? It'd be pretty hard to check if someone's "forgotten a memory deeply significant to them".

Keep in mind that as the author, you know the entire story, but the Foundation needs to have discovered what it knows about the SCP through observation and experimentation. How would someone with no prior knowledge whatsoever of the object, using only experimentation and observation, come to all the conclusions you've outlined above?