From the below description of Jean-Pierre Foissey's activities one evening, can you guess what he is doing and why?

Mr. Foissey waits until sundown. Then it is time to move. A friend whom he employs drops him off by car near the plum orchard, turning off the headlights as they approach. Mr. Foissey and another operative move quickly through adjacent cornfields and enter the orchard, careful not to leave footprints. Armed with a flashlight, his associate crawls through the orchard reading aloud the numbers on labels attached to the trees by the grower. Mr. Foissey picks leaves off the trees and marks the tree numbers on them. He takes those leaves back to an expert who will examine their size, shape, color, and texture, and also test their DNA.