How likely are staff or other students to spot a guerilla student and what would they do about it?
Does anyone care in the first place? Would it even be suspicious to just go take a seat and not act completely retarded, or do students regularly hop into random lectures that might interest them so that no one will notice?

Criminal charges are not likely to be a problem at a public university in Europe, but getting banned from the premises is a possibility and that is a bit worrying for the future.