i think you reached the right conclusion. you may have triggered a dispersal mechanism. sometimes a change in the weather or a front coming through activates these guys to "migrate to the next pool". anyone who's been in the woods when these events occur will simply be amazed at the number of animals on the move. i ride a bike path almost daily, and never see any snakes. then in one day last fall, i sighted 5 large black rat snakes along the path, coinciding with a front. maybe next time, leave his fav perch uncleaned to keep his smell there, then clean it the next week.