There's a weird sensation of something sliding into focus, and suddenly all the things about this man that resemble the Casey Don knows become glaringly obvious.

"Mike's Casey?" No. Wait. He was told something about this. "You're a third Casey."

Meaning there's a third Donatello. Or maybe a fourth, if he's understanding his brothers' stories correctly. This just keeps getting weirder...

The question about who he knows - and implicitly, where he's living - he avoids for now.