There were many people that wandered throughout the TARDIS that Sexy could have easily hidden from. The Doctor, the only other person who could navigate through the dozens of corridors and rooms without getting lost (well, most of the time, Eleven couldn't deny that he'd gotten a little jumbled when wandering around before, because the TARDIS was always changing up on him) was not one of them. Once he'd discovered that Sexy had up and vanished on him, Eleven couldn't help but wonder where she had wandered off to. Sexy wasn't like the others. She was still newly human, all tangled up and confused on the inside, and he often had to keep a close watch on her to make sure that she didn't get herself into too much trouble. The Doctor didn't mind her wandering about the TARDIS to her hearts content, but when she vanished for too long a period of time, he got to worrying. Worry led to uncertainty ; that uncertainty led to the Doctor feeling unpleasantly grumpy, because he really didn't like being unsure about his own TARDIS, trapped in the body of a human or not.
Since Eleven had been tinkering around with a few odds and ends in the main control room, he knew for a fact that she hadn't slipped out the front door. He would have seen her. That meant, of course, that his human shaped TARDIS had instead gone and ventured out into...well, herself.
It had taken a little while for Eleven to find her. Unlike Amy or Rose, who the Doctor knew he'd likely find in their rooms rather than anyplace else if they weren't with him, Sexy was unpredictable. He'd checked her room - twice - at first, just in case, then moved on to some of the other rooms, finally picking up the sound of her voice in one of the far off archive rooms that Eleven admittedly hadn't visited in years. Thumbing the edge of his bowtie curiously, Eleven poked his head around the doorway and spotted his TARDIS examining the ceiling with a sort of far off look dancing in her eyes. She wore that look often, he had noticed, and he couldn't help but wonder what was behind it half the time when she did.
"There you are," Eleven started, slipping around the door and moving inside, "I was wondering where you'd wandered off to. I just spent the past half hour trying to find you after I realized I was talking to myself. Again." Nasty habit, that.