If you can, sprinkle salt or baking soda and leave it there. Eggs cannot hatch in a dry environment like salt. It may look gross but it should stay there for a day or two before vacuuming and adding more. A single flea sighting probably means nothing if the cats are on preventative medication (advantix, proline etc) bc if they bite they cannot lay eggs. You are probably seeing an adult hatched from an egg before you began treatment or it may have attached itself to you from outside your home.