As suspected, it's Ariel who's laying. And we have proof! :)
It makes sense as she's the only hybrid we have - they've been selectively bred to lay more eggs and for longer, so no surprises really (although it does seem early to go into lay - maybe the freaky weather has had an effect).
The droopy nest box cam actually helped here!
Shortly after the egg appeared, Ariel left the nestbox:
And here's one of Renee looking cute even though she's not laying any eggs :)