This is obviously a very personal story so I'm not trying to shit on it, but isn't "sneaking in" just sitting in on a class? Professors don't care, you usually

This is obviously a very personal story so I'm not trying to shit on it, but isn't "sneaking in" just sitting in on a class? Professors don't care, you usually don't even have to ask, but if you did it seems like most people welcome it. I turn up for talks at schools I don't attend, its not like there's security at the door.