It could be the time of year. It's pretty cold outside and I imagine cats finding somewhere warm to huddle up until they think it's quiet at home and then head back.
Just been to stay with my Mum for Christmas. Next door had an ancient cat that stayed out at night (it couldn't get back in). My Mum could hear it wailing all night, so she put a box with a blanket at the top of the stairs by the back door for it to shelter in. Worked a treat. Sadly last week it crossed the Rainbow Bridge. On Christmas Eve she went out the back door and there was a different cat in the box! We wondered if it was looking for its friend.
I know cats have a territory, but I'm sure they meet and greet friends! 
Unused rooms, heat storage and heat transfer
Did you think you were poor growing up?
