I had a nightmare experience a few years ago in Corfu. Was on holiday as a single mum, my daughter had brought her friend along too. We were having a rough time of it due to the weather anyway. Half way through the holiday, we were sitting in the room playing travel monopoly when a man just unlocked our door and walked in!! I grabbed the nearest thing to hand(my daughters oar for her dinghy) and told him to get out.
He backed away and said he was in the room to repair the bath. He looked very shocked. So was I because we didn't have a bath and i'd never reported any repairs.
I reported the incident at the reception after he'd legged it, and they were very good. The rep from Thomas Cook also helped. It turned out that this man didn't work at the hotel.
But he had a key!!! I think he might have been an ex-employee trying his luck.
So now I take a small peice of wood and wedge the apartment door whenever i'm in it. I'm so security consious now. Some doors you can double lock from the inside. If i'm on the ground floor, i'll also wedge the patio door shut too. And I permanently borrowed a Travel Lodge "Do Not Disturb" sign which works a treat with enthusiastic maids.