What causes these holes in my husbands t-shirts?

A lot of my husband’s t-shirts have these holes at the bottom. Sometimes in the front, which made us think that maybe it’s the zipper and sometimes in the back (but it’s always on the side, never in the center).

Our thoughts:

• maybe in the laundry it gets caught on something (but then why do my clothes looks fine?)
• maybe it’s the Glock that he’s carrying on the belt? (But the holes also appear in the front…)

We would love to solve this mystery!!!