They don't know. I went to Cambridge, and at my college they made us sign a stupid form at the start of each term saying how many nights we'd spent away the previous term. I just did what I pleased and then lied on the form. As far as I'm concerned, the rule about 'keeping term' is completely patronising; it's none of the university's business if students choose to spend every weekend away from Cambridge, as long as they keep up with their work, especially when the university is dealing with young people who are legally adults.