Coolant Warning; No CEL codes
 

I'm getting the big red coolant warning screen as soon as I start my car.

1. No CEL and no codes from vag-com
2. Coolant level is perfect, no coolant is leaking as far as I can tell

What are some other possible causes besides the obvious (e.g., stuff I learned from searching)?

I've ordered an ECT sensor just in case I need to replace that, but I would have expected that to throw a code.

New sensor did not fix it. What else could be happening?

Could it be the wires to the sensor or any other coolant temp sensors?
I know when I was doing work on my car a while back I unhooked the sensor on the overflow and forgot to plug it back in. When I started it up, it didn't throw a code but just flash the same message.

Solved: make sure the coolant cap is screwed on all the way tightly. I was refilling coolant and I must not have tightened it all the way.