NMU Thomas Fine Arts building

There's a long hallway on the first floor (not the tower part, the long bit near Jamrich), which is lined with lockers and classrooms. There are also several bathrooms there. Some are gendered multiple bathrooms, but about halfway down the hall there's a marked unisex, single-user bathroom.

The building is generally open during school hours (between 8a and 9p on weekdays, during the school year) for sure, I can't speak for other times. Pretty much as long as somebody figures you're a student and/or staff member, or a member of the community who has come to see a show at the Black Box theatre (whether you are or not) you should be just fine.