It was far from usual for Theo to be so detached from his surroundings that he didn't notice someone moving close to his personal space; for years at school and at home he'd been as constantly on watch as a cat in a kennel, berating himself if he let his guard down even when he should have been able to feel relaxed and at ease. Even now that he was living with some of the people he trusted the most he still had a thrumming nervous tension to him that made it near impossible for him to be startled. It wasn't as though he was relaxed now though: his trip home had triggered something in his early history before he'd been honed into the paranoid and hypervigilant wire he'd been when he'd started Hogwarts, an extinguished kind of inevitable helplessness, the kind that came with the acceptance of a life with no control over when you'd be hurt or why.
After the end of the War and before the Janus Thickey ward, Theo had dissociated so hard and thoroughly into himself that he'd been all but catatonic, a machine in standby mode waiting for input now that the next command wasn't likely to appear. He was in the neighbourhood of that state of mind now, the raw immediacy of the memories he'd recalled and reexperienced back at the Nott House such that he'd needed to withdraw to safety the only way he knew to be reliable.
He didn't need to slip away somewhere else in his head now though. Blaise's voice was one as close to safety as Theo could get without resorting to drastic psychological shifts; it meant that he wasn't there anymore, it meant 'Hogwarts' and 'Slytherin dorms' but more than that it meant 'friend' and even 'trust'. Blaise didn't hurt him. Blaise wouldn't hurt him. Even when taking advantage of him would have been the easiest thing in the world at that moment.
He enveloped the fragment of bone in the safety of his curled hand, his fingers still at his mouth and his arm around his knees as Blaise coaxed his attention back to the present. He was tired with a weariness that had nothing to do with physical exhaustion, and days of solitude in the origins of his most traumatic moments had left him dazed with this sudden social contact. He hadn't heard his name but he flinched when Blaise spoke the word 'home'. It took him a beat to register that he'd been missed. "I'm sorry." Came out of his mouth, unbidden. He'd lost some of his tightly gripped control over himself while at the House too, or he would have opened with something that wasn't so... honest.
"I couldn't stay. I know you wanted me to." he frowned, looking down at his closed hand and the piece of bone it was hiding, his thoughts unmoored and left to drift. "It just called me back like migrating birds. I had to take the monster back with me. I couldn't let it stay here." Not for the holidays. He drifted off into thought again, the piece of his own skeleton digging into his skin. It didn't seem as though absolutely all of him had come back from his time away.