double take 
noun
A delayed reaction to something unexpected, immediately after one’s first reaction: Tony glanced at her, then did a double take...
- Something caught my attention, I forget what it was, so I did a double take and stepped back a few paces to get a better look.
- Half the people you meet will do a double take and make gestures at you, hinting that you've got something in your eye.
- He looked at the moonlight pouring in from the stairs and did a double take as his eyes caught something.