There's a stud-finder app? How the bleep do they do that?
Android phones and even some tablets like the Nexus 7 have a magnetrometer which can sense metal.
I have an app that allows you to alter the message it displays when it finds metal and mess with iPhone users telling them it's an iPhone sensor and when it gets close to their batteries or something it displays a message like "DANGEROUS PROXIMITY TO FLAWED TEHNOLOGY!" or whatever message I'm in the mood to have it say at the time ;)
It also can detect the metal hooks in bras, zippers, etc. and can get you into all sorts of trouble if you aren't careful.