Safe/discreet ways to flag that something's wrong

Occasionally in the news or on social media I read about schemes which allow people to flag that something is wrong in a discreet/coded way, so that even if they are in an unsafe situation they can get help without putting themselves at more risk. This seems important and I don't know how prevalent these schemes are but I think we need to make sure they're in place everywhere and people have awareness of them.

I thankfully don't have experience of domestic violence but I imagine that if I were ever in a situation like that, I would need to feel assured that I could seek help without inadvertently escalating the dangerous situation.