Identifying window that closes as soon as it appears

For a couple of weeks, a mystery window has been appearing on my screen, and immediately closing again before I can identify it. I haven't been able to discern a pattern to it yet.

I've scanned my computer with MS Security Essentials and MalwareBytes, and it's come up clean - doesn't look like malware. But I'd like to identify the program and deal with whatever it's trying to tell me. Any idea how to go about this?

I'm using an up-to-date copy of Win7 Pro.

Thanks!

I think I've got it. It appears to be Outlook's automatic send/receive dialog.

I recently added an IMAP account to Outlook for a website I manage. Since my own email is on an Exchange account, there's no send/receive dialog or interval. And since essential zero mail actually comes to the new account I added, it was one of those add-and-forget things. By the time I noticed the window popping open for a fraction of a second, I had already forgotten about that change in my setup.

Once I established that the window was appearing on a precise 30-minute schedule, email seemed the most likely culprit. Shutting down Outlook prior to the next scheduled interval confirmed it.

Thanks to A Guy for taking the time to consider the causes and reply.