Mine is that I check out people in the movie theatre to make sure they aren't "bombers".
If someone is sitting by themselves, or if they come in after the lights go down, I check out everything they have with them, and look them over to make sure they are "okay". If they get up at any time during the movie, I look to see if they are taking their stuff with them, or if they are leaving a bag or purse behind.
I know it's weird. My dh just found out about this a few weeks ago and he laughed so hard that people were turning around and asking him to be QUIET.
Anyone else have any dark secrets locked in their brain?