'Egg-Maculate' Conception? Stray rooster, no question whatsoever. You don't have to hear neighbor roosters or even know if any are around, because they migrate. Distant, backwoods farm here. We've had early a.m. migrating roosters show up for decades. Then, off they go. The natural will to reproduce has no limits, even in chickens. I've seen stray roosters, alone in the woods, but never in cold winter with snow. So depends on what time of year this took place.
Interesting. If you free range them, my guess is stray/feral roo. If he lives out in the woods, maybe you don’t catch him crowing so he doesn’t attracted unwanted predator attention. Waiting to see answer on egg color as well. You also may think no one comes into the yard/chicken enclosure but there is a still a chance someone snuck back there without your knowing and slipped eggs under as a prank. Also haven’t seen this mentioned, but it’d be crazy if she ended up hatching eggs from another bird species (randomly went out and sat on some turkey/quail eggs...other ground nesting birds)