And the answer is . . . . .
It's a low-profile two-way radio antenna. From the size, definitely NOT VHF, depending on scale it is probably 460 MHz UHF but could also be for the 800 MHz band. Inside the radome ('plastic' cover) is a printed circuit board or metal plate that looks like a whip bent backwards into an "L" shape.
I've mounted the VHF version (about 3 times as long) on a couple of coaches and some transits. The VHF version has a tuning piston screw and locknut inside the roof to tune the antenna to the part of the band your outfit runs on. The UHF versions may be broad-band, and not need the tuning.
If you look inside the bus underneath the fin, you should see a connector socket on the underside kind of like the antenna connection on the back of a CB set. Of course, if the underside is sealed up in the roof after installation, it may be why they left it behind.
No use spending $150 in labor to retrieve a used antenna that lists for $85 and probably wholesales for half that . . .
Still, the other answers you got were pretty cool . . .