Lucal chrystal violet can help unseen footprints appear. If he sees a partial footprint, he knows there is probably more next to it, but can't see it. The LCV stuff makes it show up, has a fixative in it that makes it stop after it has developed the unseen footprints.
Also cut out the whole footprint. Cut the floor and all. That's why they waited until the end of processing the scene, because its a big, messy process to cut out the pieces of floor with footprints.
Correction: Leuco Crystal Violet