http://www.staysafeapp.com

When the time is up, you have to put a PIN in to give it the okay. There's a duress PIN, so even in the scenario where a kidnapper forces you to disable the app, it still achieves its goal. The alert is triggered from the server, so even if they smash your phone it still works (albeit without any more location tracking, obviously).

There's ways to handle most worst case scenarios, so long as you accept the possibility of false positives.

Without Kitestring, emergency isn't contacted.

With Kitestring, emergency isn't contacted in your particular situation, but might be if you fell when hiking or somehow got yourself electrocuted, or were caught in a rip while swimming, etc. Or if you had a PIN lock on your phone.

Sounds like an overall gain to me.

Instead of texting "Are you okay, please respond [this way]", there's a very simple challenge-response mechanism.

Agent in the field knows the correct response and a duress response to literally any pre-shared question ("How many eggs do you need to make an omelette?" Correct answer: "I'd rather have waffles", duress answer: "three").

All of the "what if [technical fault, stupidity]" is solved by escalation. Replace a text with a phone call in two minutes. Then call an emergency contact in 5. Then call the police in 10. (Adjust numbers to your locale and situational urgency).

Don't most people have their phone locked?