Spoilers - Hint in The Dentist that shows what had been done to Brooke

When Alan greets the police at the beginning and later on when they burst into his house, if you look closely enough, you can notice a small jar of baby food on the kitchen counter, next to Alan's pills. That gives a subtle hint of what Alan had done to Brooke the night before and also gives the police a clue. It took me a while before I noticed it. I always wondered what the cop had been licking beforehand...