This morning on drive to work. The "Flat tire" symbol on my dash began flashing slowly. No "Low tire pressure" message. I went to a Gas station to check my tires and air them up. I noticed that my tires were fine. I got back in my car and as I drove away from the Gas station, the "Flat tire" symbol went away. Any idea what this may be????

Probably a dying battery in one of your sensors. The gen4's don't give a specific "low battery" code like the gen5's, but if you scan the BCM, you'll probably find a past "no data" code (C1708~C1711) for the sensor causing the issue.
C1708 - L front
C1709 - R front
C1710 - R rear
C1711 - L rear
There's information about your system in the SM here:
https://www.nicoclub.com/service-manual ... 1%2FWT.pdf

Thanks! Will scan it with my pocket OBD when I got off work.

You're most welcome. Pocket OBD's won't generally detect the BCM unless they're specifically TPMS-capable, but you're not stuck with a dealer to scan it. Most tire places have TPMS-purposed scanners and most good shops will have high-end scanners that can see the BCM.