He knew something was up, he'd never felt this way before, like someone was constantly observing him. It wasn't malicious, he was able to figure out that much, but he had plans for you, you just didn't know it yet. He'd been purposefully acting oblivious in order to encourage your presence. You were like a cat, only coming out when you were certain he wasn't watching.
He noted you were respectful, never watching him while he was in the shower, or watching him while he 'slept'. You just seemed curious, and he could respect that. he spent a whole year familiarizing himself with all the signs and traces you left behind. He found you charming, you'd clean the house while he was away, take care of his dog, Mochi, and were overall helpful, so he couldn't help but become attached to you.
It was a complete accident, he'd left his computer on with his plans to go on a trip just sitting in plain view, fully intending to catch you off guard, but he hadn't expected this. Well know he knew, ghosts could shower. "Ah...so this is why my energy bill's been higher than usual lately..." He comments, an amused smile on his face. It was the first time he was seeing you, the way your skin glowed a transparent blue made it clear that you were but a soul. "Shouldn't you introduce yourself?" He asks after a moment, gazing down at you curiously.