Answer: "Turns out it was a family member’s lipstick that got on the key while it was in their purse."

Answer 2: "So this happened one time when I was flipping houses for a dude. Went to unlock the door and noticed something covering the lock that looked like white lipstick. I looked around, and the other houses had it too. I called crime check, and they said it had been used as a method by squatters/burglars to see who on the block is using their key or even if there might be empty homes. Like how people sometimes go door to door pretending to sell solar panels, window replacements, or landscaping services, so they can see who answers in the middle of the day."