Yeah the staircase area is one of the more consistently documented cold spots in the whole building. I did an investigation there back in 2022 and we had a thermal camera running the whole time - the temperature differential between the main corridor and the base of those stairs was pretty significant, something like 8 degrees across maybe two metres of space. No obvious air movement to explain it either, we had an anemometer running alongside.
What gets me is how localised it is. Cold spots caused by drafts or poor insulation tend to bleed out gradually, but this one has fairly hard edges if that makes sense. People always jump straight to "ghost" but honestly the more interesting question is what's causing that kind of thermal boundary in the first place.
Did you notice if it was more pronounced on certain floors of the stairwell? Someone in another group I know said the fourth floor landing felt noticeably different to the lower levels. Would be good to compare notes if you've got any readings.