As another customer stated - very temperamental. You have to be close, whistle fairly loudly and then it may still not go off. However, letter coming through the door can set it off without any problems, or maybe if you're playing some music nearby. Either way - steer clear!
bought as a christmas present ended up keeping it for myself. They say that gadgets with only one use are on the way out...well i hope that includes the whistle finder as it doesn't even do that well. Goes off in my pocket, goes off listening to music in my car, goes off in work when in my draw. Then of course when i'm looking for my keys i almost pass out from over-whistling before i actually remember that i left them in my coat pocket all along. Stay clear!