Three nights in a row is actually significant - that kind of pattern almost never gets mentioned in throwaway hoax posts, so I'm inclined to take this seriously.
A few things worth ruling out before jumping to conclusions:
Environmental factors first:
Infrasound from HVAC systems can cause visual disturbances and a strong sense of presence. Anything running at 18-19Hz in your building?, Carbon monoxide at sub-alarm levels. Seriously, get a detector if you haven't already - low-level exposure causes exactly this type of visual phenomenon, Pareidolia from headlights passing outside
That said, hallways are genuinely interesting liminal spaces from a paranormal research standpoint. I've done overnight sessions in two reportedly active locations here in Nevada and both had consistent shadow activity concentrated in corridor areas specifically. There's something to the geometry of narrow enclosed spaces.
What I'd want to know:
Did it appear at roughly the same time each night?, Any temperature differential you could feel, or just visual?, Solid mass or more of a peripheral thing that disappeared when you looked directly at it?
The peripheral-vision-only type is almost always explainable. A solid, direct-view shadow figure that maintains form when observed directly is considerably more interesting and documented far less frequently.
If you've got a spare camera lying around, even a basic Wyze Cam on night vision pointed down the hallway would give you something concrete to analyse. Document everything - timestamps, your own physical state, weather conditions. Pattern recognition across three data points is where this gets genuinely useful.
What's the construction age of the property?