Each year, a few mason bees set up home in the fence posts here. I think that they are Osmia rufa (the red mason bee, also known as Osmia bicornis).
They’re very fickle about which holes to choose for their homes, visiting all knot, nail and screw holes before selecting the one to use. Invariably they use the same few holes in the same few fence posts year after year.
Once they’ve moved in, they will sit for hours in the mouthway of the hole, seemingly doing nothing much at all. They’ll be around for a few weeks, and then as suddenly as they arrived, they’re gone.