We have two young cats (2.5yrs), litter-mates and brothers. They're very different! Big Boy is very bonded to us, comes when he's called (it's mostly to do with food as he's a slave to his belly), follows us around in endearing manner if we walk down the lane at the back, etc. His brother, Little Boy, is a feral-hearted little bastard but we love him dearly. He's about 2kg lighter than Big Boy and doesn't live for food, tends to come in to graze a mouthful of biscuits and then buggers off again for hours. He ADORES his outdoor life. We live semi-rurally and I'm afraid Little Boy goes after voles, etc. He eats the entire thing systematically, it puts me in mind of a lion settling down to devour a gazelle on the veldt.
We've got a cat flap and have always aimed to get them both in before dark, but this is almost impossible with Little Boy these days, who is a creature of the darkness and very wary of being caught and brought in. So we've done a fair bit of calling around in the dark with a torch, looking for the gleam of his eyes.
His late nights and all-nighters are becoming more frequent. Three times in the last week he's not come in for 24 hours. He's AWOL at the moment and it's been about 30 hours now. 
I really worry that his disappearances are stringing together closer and closer and he just won't come home one day, at all. (When he does come in, he has a bit to eat, I check him over in case he's had an accident, and then he just has a very long nap, sometimes most of the day, and then he's ready to go again!)
He used to be extremely clingy with us as a kitten and young cat. He'd sit on my chest or shoulder and I'd carry him around like a baby - he seemed to love it. Now he seldom seeks cuddles, though (on nights he's actually at home) he does prefer to sleep on our bed or in one of our bedrooms. He's a strange little thing. You go up to him and stroke him while he's curled up and he won't purr. We joke that he's made of stone. His brother will start up the purr engine immediately.
Could it be that he just isn't that into us? He really used to be.
Regardless of that, I'm worried silly about him. We don't think he's seeking affection (and food!) elsewhere in the neighbourhood, though you never know. He doesn't smell of anyone else's house or perfume, just of leaf mould and vole gore. We can't keep him in, it would make him utterly miserable and be almost impossible to manage with his brother.
Anyone else have a cat who's decided to break the bonds a bit? It makes me so sad.