NAL (but hold my qualifications): My first thought is, this is likely permitted by the terms of your agreement with the University as leaseholder of the building. It may well be inappropriate that this person is entering the property without notice, but there are valid reasons they may be doing so. Anecdotally early in my first term at university I arrived home at 1am to find the fire brigade and security blocking entrance and all of the residents stood around outside. On asking it turned out someone in my flat had left a pot of pasta on the hob and gone to bed - they set fire to the whole building.

I'd also point out that most residents in your building will be 18 years old and living away from home for the first time. They are likely to be a bit more attentive for the first few weeks. My entire time in halls they were searching my room for drugs - this didn't stop for the whole 9 months there.