Tinks, you could put one of those window type alarms on the door - they can be just stuck on with stick pads. As soon as someone breaks the seal, it goes off. May alert anybody else in house, when you are out that this person is entering your room.
I'm not sure locking your room would go down well with the homeowners, but maybe a lockable chest of drawers or something that could hold the things you want kept private?
Not a good idea Eddie, if it is put on the outside some clown could easily lock tinks in the room & if it is put on the inside it could create a fire hazard.
Can you borrow or hire a video camera & hide it in the room hopefully in a bookcase or similar & then you will find out who it is & please let us know the outcome.
Tinkles ............ assuming you have standard doorknobs/levers with a tubular latch. ...............
This should be a direct replacement..............
When you're in all you need is a door wedge, usually used for keeping them open but very effective and no fitting required. You can take it away with you and use it in hotels too.
I'd only ever fit locks for owners, not even tenants without permission as could then be done for criminal damage by landlord.
Union 1148 is my recommendation for all the students though.