The idea here is a brand of baby monitors that occasionally broadcasts sounds of the baby in extreme distress, however the baby is totally fine and there are no ill effects to it whatsoever. The effect only happens if there is only one person listening (perhaps the mother) and immediately stops once that person moves out of hearing range, e.g. the mom hears the baby crying over the monitor and runs out to help only to find nothing wrong; when she returns, the baby monitor is functioning as normal and she thinks she's going nuts. Sometimes the monitor broadcasts sounds of people abusing the baby (e.g. a man drunkenly telling the baby to shut up etc.) and sometimes it's just the baby crying on its own.
This is heightened (and comes to Foundation interest) when the monitor starts prefacing its anomalous transmissions with a mysterious man's voice saying that the mother has to sit and listen to the whole broadcast or else bad things will happen. As before, if the mother ignores the command nothing bad happens to her baby.
The twist happens after this occurs multiple times: One mother obeys the starting command and listens to the monitor for several minutes; she then hears a different man threaten the baby and recognizes him as a guy who lives a few blocks away. The Foundation investigates this guy and he hasn't done anything to the woman's child… but he has been abusing his own baby. The reveal is that this isn't supposed to be something to terrify mothers but rather a sort of naive distress signal – the force responsible for this anomaly was trying to alert people to real abuse, and chose baby monitors because it figured that mothers who cared enough about their children to want to keep an eye on them remotely would care about these abused children as well. The mysterious man wasn't trying to threaten the mother but rather to compel her to pay attention so that the abused babies can be saved.
Of course the obvious point here is “why wouldn't the mysterious man just tell the mothers that they're listening to someone else's baby?” I don't have a great answer to that, apart from that it's what the story requires to not give away the twist. Instead of actually having a voice I could make it a compulsion effect where the mother is forced to listen to the full transmission but I feel like that cheapens it somewhat – I think it's more awful if you know that the mother could end this at any point by simply disregarding the instructions and running out of earshot, as opposed to making it so she literally cannot move.
Finally, I was thinking of making this a loose sequel to an SCP I co-wrote, SCP-2453. It's been reasonably well-received but definitely isn't an SCP classic that people would immediately be familiar with so I'd probably just reference it with a few lines saying the branding on the baby monitors is consistent with SCP-2453 and that the mothers affected by these baby monitors tended to have the same characteristics as the “good mothers” affected by SCP-2453.
Any and all feedback on this idea is welcome.