After looking for the signal and not finding anything and thinking back on what I did hear I now believe Richard Kramer had it right. A FSR radio left in the police car with VOX on would be consistence with what I heard. I only had half of the transmission. The signal lasted a couple of days (about what I would expect from a good battery pack on the FSR) That or it was found by the cleaning crew, or the kid got all worked up over the lost FSR radio and Daddy found it in the car. DOH!
I always have the GMRS scan running as it only takes a fraction of a second so if the signal comes back I will try and investigate but I believe it was a FSR radio left by mistake.