Either he goes back to "clean up", or the body is discovered and he realizes that there may be something at the scene linking him to the crime.
I thought it was pretty good for a season premiere, which are usually pretty boring as they set things up for the future. Dexter's inability to express much emotion making him look suspicious was great, and so was that second where you almost thought he'd make a run for it (well, if you didn't watch the trailer for the season).
Can't wait for the next episode.