I do one of two things when this happens to me. I walk back into the house the way I did that day. This may help to refresh your memory. The other thing I do, is to fall asleep thinking about it. I wake up in the morning and it's the first thing on my mind. I can walk right to it. I need a well rested brain to make up for the tired one.
I like PenniF's way of thinking, too. One of my friend's found her mail on the water heater, downstairs, once. She had no clue how it got there. Good luck! I sure hope you find it!