Icon flashing in taskbar

For about the last several days, about once every 15 minutes an icon briefly flashes in my task bar. It appears to be the same color as Outlook’s icon but it flashes so quickly I’m never looking directly at it when it flashes.

I’ve checked task manager and Outlook isn’t running. It’s as quick as a snap of the fingers. I’m running Windows 7 Home Premium and my task bar is setup so running apps are the small squares and that is what is flashing.

I’ve scanned my PC for viruses and am very cautious about the websites I go to and am fairly certain I don’t have any infections. Although we can never completely rule that out.

Does anybody perhaps have an idea what this might be?