Default [Q] App that warn me when someone arrives home

I'm telling you my problem:
When I'm alone at home having sexy time with my gf we are always worried about my parents coming home busting us.

So I'd like to install in my parents Androids some app that sends me and email when they are near the house.
I don't think something like that exist, but I think some kind of tracer in combination with some app like IFTTT can do the job.

Needless to say that they shouldn't know that this app is installed. My parents think their smartphones works on some kind magic so if I can hide notifications it's enough.

Can you help me? Thanks you all!

Sorry if my english is bad.