I came home from work yesterday and Milton greeted me at the front door.
I brushed it off as my own forgetfulness - I must have somehow forgot to put him back into the yard after lunch.
This morning I let him out again and what do you know - 5 minutes later he was back in the house.
Put him out again and watched what he was up to.
Turns out he has finally worked out how to use the cat flap. Yes, we have a cat flap at home and Milton is small enough to squeeze through it. Only took him a year to figure out how to use it.
I suppose he is well-behaved enough to be trusted indoors even when we're not at home now... so I guess this was his way of letting us know he wants to be an indoor dog.