No clue

No, they certainly don't lay out in the open with their mouths open to cool down.. That would be reptiles such as alligators/crocs and Beardeds. Snakes do no such thing. If a snake has its mouth open for a period of time(longer than a normal yawn), it is trying to breathe.

Becky you must have misread what I wrote. I'm not saying snakes lay with their mouths open I said it's similar to the purpose of what crocs do. But excessive yawning can indicate they are too hot and are trying to cool themselves down. Not saying it's the case. But it happens.

I'd say it may be RI, up your temps and humidity, and make an appointment with a vet.