How about implanting some of the more stolen signs with RF tags. Like those little things in your DVDs that make the alarm at the exit go off at teh front door of the store. Anyone passes through a door with them sets off an alarm and, hopefully, the staff responds.
Those tags have to be cheap or else they wouldn't be in so many products, the readers might not be, however losing 200K worth of signs might make the money worth it.
I think paying attention might also be a good idea. Heck, for the money they're losing, hire some undercover personnel to hang out in the stations and keep an eye on things. Sign theivery might not be all they catch.