Ok first — it’s not a guinea chicken, it’s either a Guinea Hen (female makes a two syllable buckwheat sounding call) or a Guinea Cock (males make a three syllable very shrill call)
Second - it did probably lose its flock one way or another, they are miserable alone, should not be raised with chickens because of their breeding practices, they can get very aggressive and rough with chickens, even mine that WERE NOT raised with chickens still like to sneak up on my chickens and pull a feather. They’re kind of jerks. They prefer flocks of 10 or more to be happy.
Third - dear god don’t open a window. I could NOT imagine having one of my Guineas in my house. They’d break everything, freak the everything you kNow what out. Best to build a cage, lure it with treats (meal worms, white millet, something it likes, may take trial and error) - the one thing you’re right about is being easier to handle at night, they’re completely blind, so if you got it in a contained area, theoretically you might be able to grab it, watch the beak and toenails, they’re sharp as knives.
If you do manage to catch it, please find someone with a flock to regime it - but honestly, if it’s being left in the elements and doesn’t have shelter, don’t expect it to last long.
@R2elk... care to add if I missed anything?

Auto correct, I would rehome it rather than regime it.