Yeah, I got the impression that this was what was going on myself. However, worth noting that the guest parking voucher has limitations; you can't use it to park in front of a place forever. (I forget the exact restriction, but there's a time limit.) So if the car was using a guest permit, but was using it for months at a time (e.g. while living in that apartment), then yeah, a ticket is legit. Guest permits are for, well, guests.