Not all cars require you to press the brakes to start.

That depends. On most modern vehicles, the front brakes do the majority of braking. Cut the rear brake line and you may get several presses at what feels like adequate pressure, but when the master cylinder runs dry and sucks in air past the piston, no more brakes. Who knows without being there, but it's definitely possible, especially for someone that doesn't drive often or doesn't notice a pedal feeling slight off. I have had people come in and hand me a brake pad that fround so thin it slipped out of the bracket. People can be oblivious sometimes.

If you park on the street, and it’s level, might not notice until you’re up the street a bit.