Alternatively (or supplementarily), there's a "Find My iPhone" app/website combo that will, among other things, let you cause your phone to make a noise even if it's in silent mode. (Assuming that it's on and has some charge and some connectivity, but for the usual "I left it in here somewhere" case that's likely.)

It can also display the phone's location on a map, which is handy for the "I'm at work and I don't have my phone -- did I leave it home, drop it somewhere here, or something worse?" case. (Most recently for me, "my iPad's not in my bag" because I'd forgotten to unplug it from the charger and put it in my bag....)