I've lost my iPhone without Find My iPhone app installed?

My mum took away my iPhone because she was angry at me, it's only an old iPhone 3GS and of course, she goes and looses it. We have searched EVERYWHERE and it's not where she usually hides it. Normally I can find it because she hides it in really dumb places, but this time even she doesn't know -.- I didn't have the Find My iPhone app installed on it and I'm worried that means I won't ever be able to find it… Please help! If there's any last chance I might be able to find it… Thank you!

Added (1). It has definitely run out of charge by now… So I can't even call it…

The best thing you can do is try and call it, and make sure as soon as you find it to install find my iphone.