They do make these electronic systems. We use them in baby nurseries (we call them baby lojack) and they are used in many elder care settings. It's called WanderGuard. Right now I haven't seen them for home use but I'll bet there are some devices out there for that.
Here is an info page on WanderGuard
http://www.seniortechnologies.com/co...m/04WG%20D.pdf
Russ