... and I can't think why
He's 6, short haired and he doesn't normally love the cold. It's January. My garden is a concrete wasteland low maintenance landlord's garden where there's so little wildlife that even my bird feeder went untouched. Theoretically it could be a rat, but he's fairly ruthless in his pursuit of them, so I doubt it's that, or any other hunting.
He's taken to intermittently going out into the garden and point blank refusing to come back inside. It's often dark when he does this, and it's hard to see what he's up to, but I think he's just standing / sitting there. On one occasion when it was light, he was standing outside in the drizzle, shivering and refusing to come inside - I had to pick him up and bring him in in the end.
He doesn't appear ill in any way, and is perfectly happy to go out for a walk. No changes at home; it's a quiet home and I keep the temperature around 18C so he can't be overheating.
Any ideas, or is he just being an uncharacteristically stubborn git?