Right, but has nothing to do with it being a bank. Basically any place can inform you that it is not acceptable, and ask you to leave. That verbal notification would be the same as a sign, and then afterward, it is a trespass. If it is not posted, and they do not say anything, they cannot call police and charge you with a trespass until you are informed. It kind of sounded like you were saying that they can prosecute without notifying you. They have to either post or tell you in some way that it is not acceptable. Until then, you should assume that it is permitted.