When I'm cleaning a site I always try to take the time and walk back through the area I just cleaned. Too often I've found pieces of debris that have fallen out of my trash can or off of my dustmop, etc.
When cleaning I have found it is best when you finish an area to look behind you for anything you might inadvertently drop. It's best you find it right then instead of your customer finding it the next morning.
Hope this helps.