My brother found this at the bottom of his rice bowl.
The rice bowl had basmati rice, gyro, yogurt mint sauce, and tomato/red onion salad.
When he asked the manager what was on his bowl, the manager apparently told him that it was normal due to the way they store their onions.
I've never seen this before. It looks like the ink one sees on meats that have been usda stamped, but I can't tell.