It is possible people are vandalizing the cars
Sure, but... "new rules were put in place requiring officers to document that both antennas were in place at the beginning and end of each shift. To guard against officers removing the antennas during their shifts, Tingirides said he requires patrol supervisors to make unannounced checks on cars."
"Since the new protocols went into place, only one antenna has been found missing,"
As soon as it became likely that the vandalism be caught, the vandalism suddenly dropped to almost zero despite the fact that only the officers knew of the change.
So no... it's not possible that the public is vandalizing the cars.