If this is the wrong group I'd appreciate a pointer to the right one!

I was wondering if it is possible to detect the presence of inactive
cell phones/wifi devices by detecting radio signals they emit. I
figure wifi would be the best bet because I guess cells dont broadcast
all that much when not in use, right?

Would there be enough output to find them? Would it be strong enough
to detect over a distance of say 100m? Could signals from multiple
devices be distinguished? Would the equipment be very expensive. Would
I get thrown in prison?

I am interested in triangulating and tracking the movement of phones
through a small area as a potential postgrad project.

Thanks.