Today, after the first CNC piece passed inspection, the dimensions started drifting slowly. How can I determine that it's not tool wear?

During the morning shift today, we processed a batch of aluminum parts. The first piece and the first ten pieces were all within tolerance, but then the hole positions started drifting to one side. The operator first suspected tool wear, but since the tool compensation hadn't changed much and the chip color was normal, I checked the fixture positioning, spindle thermal expansion, and coolant…