If you had a coworker that only speak to you when you're outside the building together what would you think?

I have an employee who doesn't speak to me at all. I caught him sitting outside for four hours while on the clock. He was going for some kind of overtime incentive. The problem was everyone on my shift kept asking why he was just sitting there. I didnt want to bother him if he was just avoiding going home or his car broke down. But then i saw him clock out and leave. So i brought it up to his day shift supervisor and she couldnt believe it. She said hes usually the hardest working guy they have. But yeah, i guess he only works hard while shes watching him. I guess he blames me for spilling the beans...

Im usually fine with someone milking the clock for an extra 15 -20 minutes or whatever but four hours is nuts.