There are two things that need to be learned and practiced to take with you, that may be extremely useful.
The first is American Sign Language. Just knowing how to say and understand some critical words could prove invaluable if either someone loses their hearing or ability to speak, or if speaking could “get you into trouble”.
The other thing is similar but covert. It is based on psychology, that people tend to ignore what is happening below their belt line (which is why the US Army teaches soldiers that, even briefly, they should never hold a hand grenade lower than their belt, or they might forget they are holding it.)
It is a form of signalling that uses the feet. Importantly, it starts by doing something up high that tells those in the know to look down, such as tugging on your left ear lobe. Then you inconspicuously move a foot, or both feet, to send a silent message.
Most of these will be “duress” signals, since you are in a position in which you can’t or shouldn’t speak, or maybe even use ASL.
There might be great value in these, similar to using “duress” passwords to let your friends know you are being coerced.
Morse Code and Semaphore will also come in handy for long distances or for rescue signaling.