I'd bet there's your answer...she could very well be laying out in the range area somewhere.
Sounds like it's too hot to lock them up in the coop but:
Free range birds sometimes need to be 'trained'(or re-trained) to lay in the coop nests, especially new layers. Leaving them locked in the coop...
Welcome to BYC!
Tell us more about your flock and coop/run in general.....
....and specifically:
Do you have a cock/erel?
Do you free range?
What and how exactly are you feeding?