Dean got into a discussion with Crowley so he just went to wherever the hell he thought it was a good idea and there was no better place than yours, (insert place), he found that a good idea because he was being sought by Sam and Crowley now, but none of them knew you or your place, so he appeared there out of nowhere and knocked your door with a cocky smirk. It was so noticable that there was something off with him anyway.