Were you officially allowed on campus at all?

I'm not sure I understand your question, but anyway I went to grad school in an urban area of a major city and there were no ID checks during business hours, i.e. when classes were held. There was security to keep thieves and other ne'er-do-wells out but not to keep people from auditing classes.

In theory you did need ID to get into the libraries but that could be easily circumvented -- apparently if the library has government documents they cannot deny entry to the general public.