You can't stop looking for him everywhere you go. At first, you restrict yourself to curly-haired men with outrageous scarves, but a few months later, you remember that he can change faces, a fact that makes it so much harder. He could be anyone, anyone at all.
How can you look for a man when you can't describe him? You've left instructions with UNIT to call immediately if he ever returns, but what else can you do? Police boxes aren't exactly common either, but with all of time and space to travel, what are the odds you'll run into him again? He did say that humans were his favorite species, but the universe is so vast, and you are so small.