This is the first cold snap since our pool was completed and I awoke to the same thing two days ago. All of our equipment appears to be working so the only thing I could figure out is that a little water was moving from the spa to the pool every 15 minutes when the valves turn.

I have an automated valve on the spa spillover so I turned freeze protection on for that circuit last night. Now when it switches to "pool" it also pumps water into the spa. This morning the spa was full.

I'm not crazy about running the spillover when it gets really cold but this seems to have solved the problem for now.