I've said it before and I'll say it again: Removable faceplates (or variations thereof such as "mask" units) are totally useless. You're proof of that. They knew there was a stereo there to steal, so they stole it.

Not to trivialize your loss, of course. I am very sorry you got ripped off. It's happened to me twice now, so I know exactly how you feel. It completely sucks when that happens.