Right so you're not alone here. I've had something similar happen in my old Victorian terrace in Leamington - top of the stairs, just standing there, gone the second you really focus on it. That peripheral vision thing is well documented with these sightings apparently, something about how the rods in your eyes pick up movement and low light better when you're not looking directly at a thing.
What gets me is how consistent the descriptions are across people who've never spoken to each other. Always the stairs, always dark, always that sense it knows you've clocked it.
Did you feel anything before you saw it, like a temperature drop or just a general unease? I find that's the bit that separates a genuine encounter from eyes playing tricks - that gut feeling that something is wrong before you've even registered what you're seeing.
Would love to hear more details mate. What time of night was this and how long did it stay visible?