You're definitely not losing your mind. Shadow figures at the top of stairs is one of the most commonly reported locations for them - there's something about transitional spaces like doorways and staircases that seems to attract this kind of activity, or at least that's what I keep reading.
I've been trying to capture something similar at a property in Birmingham for months now. Never managed to get a decent photo but I did get some interesting EVP results in the upstairs hallway which lines up with where people were seeing movement.
What did it look like exactly? Was it a solid mass or more like a darker patch against the dark? And did it move or just stand there? Trying to work out if what you saw matches anything I've come across in my own research. Some people reckon shadow figures at stairs are residual, just replaying the same movement over and over, others think they're fully intelligent and aware of you. Would really like to hear more detail if you can remember it.