Meet Grandpa Gumshoe.

When thousands of dollars vanished from a drawer in his room at a Brooklyn senior-citizen home, John McGinnis, 83, folded a business card and wedged it between the drawer and the dresser. When his home attendant opened the drawer and the card dropped to the floor, she was caught red-handed.

“We went outside, and I told her, ‘You’re a little crook and you stole my money!’ ” McGinnis said.

Nikou Santos, 21, of Brooklyn, was arrested yesterday.