I used to work for a MLB stadium in security. Part of our duties after the game was to go through EVERY bathroom and open the stall doors to see if someone was hiding. We then worked our way down each level until the stadium was cleared. You’d be surprised how many people we caught trying to go back up to levels we cleared.
My two baseball friends in college snuck into dodger stadium on the 25th anniversary of the stadium. They entered through the vendors gate before the public gates were open. They hid in the toilets often squatting on the seats so that people who came in could not see their feet. The game was sold out and they spent the entire game in the standing areas behind the last row on the concourse.