There is man in his 60s at work who is regarded as a bit of an oddball. I am friendly with him as I am with everyone, but others in the office joke that he is strange and lecherous.
I brush it off but I feel slightly disturbed by an incident the other day. I was collecting a delivery so I had to go outside the office to get it. By chance I looked behind me up at the offices, and I saw the side of his face and nose looking out at me from a window which would be the photocopy room (with the other half behind the wall). He has no need to be in there in his role. He then moved and hid his face. I looked away and then looked back 20 seconds later and I could see his face again looking at me. He then hid again.
Later I asked what he was doing, he said he was looking at the delivery van. I said 'why did you do it hiding with half your face behind a wall? 
'
It has made me feel weird but idk if I'm overreacting? I don't feel at risk but the whole thing makes me feel a bit unsettled. I hate the thought of being watched as I am quite a paranoid person.