What's going on here?

Here is an interesting picture from 1992 Stadium Club. Jim Gantner is getting back to the bag on a pickoff throw to Detroit's Cecil Fielder. But who is that guy in the background?

He's clearly not a Tiger. With the full stands I don't think this is pregame activity, and the person is clearly in fair territory. It would seem to be an umpire, but since when do umps wear white hats?

My guess is that this is Memorial Day, 1991, and the umpires were wearing special gimmick hats like the players wear nowadays on holidays.