Isn't the idea here that it's subtle and easy to hide, not that it's a secret?
Secrets can be kept in the open for long periods of time if the bad actors remain ignorant. Look how long thieves were breaking into Teslas without realizing they were being recorded by the built-in cameras, even though examples of this happening were common.
I wonder what is the optimal balance between being so obscure that the bad guys probably don't know it but the victims and potential rescuers do.
It can still be secretly conveyed, unless those bad actors bind the victims' hands or somehow watch them every second.
Bad actors know about 911, and folks are still sometimes able to call it to get help. Even if well known, this would be the worst case scenario. To be honest, though, it won't be so bad: There are lots of times that someone might be able to secretly make a hand signal - and those are the same times you can't exactly tie someone's hands together. (you can just take a phone away)