Mass hysteria or actual ghost?

gas leaks and the eye playing tricks do happen but so do ghost in my opinion i think it very well could be either

Have they researched the property that the office stands on now? It could just be a gas leak as the previous commenter said. But it could also be other possibilities varying from someone dying on the property to it being a possible Indian burial ground, which you would be surprised as to how often that happens, and the boss/head honcho not saying anything. It's always just good to do some research, especially if multiple people are claiming to see the same thing, and it not being a coincidence. I have been led to believe that this isn't just a coincidence due to you saying that multiple people have seen the person/spirit in the corners of their vision, and that it's not just you're mother and her co-worker seeing it. 

There’re actually many accidents reports happened inside and
near this company area. Not sure about the burial area underneath the property
though. Also, I haven’t heard anything about seeing that female figure from
them for a while now and nobody seems to care about this ghost story anymore
(or maybe if they still see it, they just might not tell me bc they know I’d
ask them about it non-stop). Btw thank you for your opinions!