you'll have been visited because (as standard) once an address is in the system as not having a licence (and ALL addresses are in the system........any new builds once the postcode is known are given to TVL) a letter is sent out to the address - and then "visits" sent to the licensing officers/enforcement officers.
If someone visits the address and no-one answers the door then there's a code which they put on their bit they return to TVL offices - and it goes through the cycle again. Obvously unless it's VERY obvious it can be difficult to know if it's a case of nobody at home/no-one living there.
As for the halls of residence - if every room is numbered until someone visits they won't know that some of them are toilets etc......THAT instance is probably the HoR fault for numbering each one and then telling TVL that there are "x" number of rooms.....
They don't send letters every time you move. What they DO recognise though is that if someone that has a licence has moved out of the address that you're moving to - then it will automatically get sent a letter saying there's no licence. Of course because the system happens in 'cycles' this can mean that it overlaps with you calling to transfer your licence (and you'd be amazed how many people forget to transfer their licence when they move - DH does roughly one a day!).
How are they supposed to know who is living at an adress where there is no licence registered??? Unfortunatley the only way to make sure that all addresses are covered is to send letters to all of them, and follow up with a Sales Officer (or if you're bl**dy stubborn and rude to previous SO's you may get Enforcement Officers - who usually work in pairs).
Oh and another thing that could affect if you get "the heavies" or the "sales officer" is if there's been a reported incidence at the property you're at. Most of the current records go back to beginning of 2003.......so if in March that year a Sales Officer was attacked/threatened dangerously (like having knifes/guns waved in their faces) it will be on the system - and most of those visits then go to the Enforcement Officers who work in pairs for their own protection.
If you DO get a threatening visit......report them FGS - otherwise it gives the other 95% of them a bad name and they just end up getting treated worse.