Half an hour north of Philly is a free bridge into Trenton that is (1) small, and (2) as far as I know, unmonitored. Its called the "Trenton Makes" bridge, based on the words "Trenton Makes, the World Takes" on the bridge. North of that bridge are several other very small, non toll, non monitored bridges between PA and NJ. The car very easily could have simply traveled North and crossed any of those bridges.
It seems more likely to me, however, that the easiest thing would be to simply drive east into a deserted remote area...