Look at combs and wattles closely. See if you see any injuries, after 2 days it could be just a tiny black spot, or could even be gone. But even a small nick or injury to combs and wattles can bleed a lot, and if they shake their head it will go everywhere. Could have been a roosting space spat. They heal pretty fast, so often all you see is a tiny black spot.
The only concern would be if you are seeing blood in droppings instead. If droppings are normal, and as you said they are all acting fine, then likely that is what happened.
*EDIT* as an aside, I noticed loose screws on the window ledge. While they are apparently (?) large ones, I would remove those so that they don't get swallowed by a bird. I've seen them eat some really odd things on occasion, ingesting one of those would be very bad news for the bird. Swallowing nails, screws, staples, etc can cause what is called hardware disease and can cause internal punctures and infections. Honestly, anything even questionably swallowable, should not be where they can get to it. Better safe than sorry. There is a thread on here somewhere where a pullet swallowed a foot of paracord.
