I just walk in unnoticed pretty much every time I explore, honestly. I will say though if you do sneak in you are running the risk of getting arrested for trespassing and/or breaking and entering/vandalism depending on how you got into the building itself. There have been times where I’ve either run into the owner of the property or the police. If you do choose to just walk in, be sure that if you’re caught that you cooperate. You’ll save a lot of trouble if you don’t cause any.
The reason I don’t ask permission is that you’ll probably be turned down unless you’re with the news, media, etc.
Also, thank you so much!