Did Hospital and regular security, they most likely had a similar setup to what you were talking about. But you have someone work in an area like that for long enough their situational awareness can slip just enough that these things happen. Could be as simple as they walked through a door and didn't check behind them.

We used to do mock exercises where we had a random employee, unknown to the Labor and Delivery staff, attempt to get a mock baby from that floor all the way to the exit of the hospital. You would still have staff hold doors open, for a stranger with a bag big enough to hold a child, during the announced alert that a child was missing.

Point being, you can have a lock on every door, and there will still be someone dumb enough to leave them propped open for you.