Went past Briar Hill about three weeks ago, not last weekend but close enough. Didn't go inside, just the perimeter, but I had my old Olympus with me and got three shots with this odd smearing on the left edge. Not lens flare, wrong angle for that.
The "watched" feeling is interesting because it tends to cluster in certain spots rather than being evenly distributed across a site. Psychical researchers call it a "fear cage" effect sometimes - basically EM fields messing with your brain. Worth checking if anyone had a compass or EMF meter and where exactly they were standing when it kicked in.
What part of the building were you near when it happened? Front elevation, rear? Basements tend to concentrate the effect if theres water pipes or old electrical running through. Just trying to map it a bit.