chebby said:
Get your torch (preferably a good LED one) and get down on the floor with it (side of head actually on the floor). Now shine the beam of the torch along the floor and look along the beam. Even small things will show up quite well this way. Of course if it's carpeted you're b#####ed!
Well, not quite b#####ed. Cut off a length of ladies tights so you have a 'sock'. Feed that into your vacuum cleaner tube with the open end of the 'sock' overlapping the end of the tube. Now put your carpet attachment on the end of the tube so that the end of the sock is clamped in place.
Vacuum the entire floor. Now turn tights 'sock' inside-out and empty contents into a bowl or a bucket. The little screw will have - hopefully - got trapped in this rather than disappear into dust bag. (Assuming it was there in the first place.)