It's not like once everyone is gone Disney just turns the lights off and its night. it doesn't work that way. There are several guests that stay at the hotels and close to the parks. Disney has around the clock security. After everyone is gone every park get a security check through the night. The parking lots do to, Disney springs does.
In the early 80's and 90's before all the high tech passes and everything it was easier for people to sneak in to a park. Disney had actual paper tickets. Those tickets either didn't require a finger print or face picture. It was the size of a credit card and paper. The ticket had the date on and what type of it. So people would go up to the entrance the ticket to get in. I've seen this done. If you had 5 people and only 4 tickets. One person would leave the park with two tickets and give the second ticket to another person and then those people could walk back in.