Possible Apparition in Derelict Building - East Sussex - Rational Explanations Welcome

So this is going to sound mental but I'm posting it anyway. My mate Jake and I were exploring an abandoned manor house near Hastings last Saturday afternoon (about 2pm, clear daylight, both completely sober before you ask). We were doing a bit of urban exploration photography - nothing illegal, just documenting old buildings.

In the main hallway, we both saw what looked like a figure in Victorian dress standing about 20 metres away, very briefly. It was there maybe 3-4 seconds. We both saw it at the same time and both reacted. Jake said 'Did you - ' and I said 'Yeah I saw it too.' Then we looked back and it was gone. No one was actually there when we checked the room.

I know how this sounds. Pareidolia, trick of the light, old building creeping us out. But we both independently saw it at the same time, same place. Would love to hear proper theories about what could explain this besides 'you imagined it.'

This is actually a well-documented phenomenon in exploration communities. Old buildings have terrible light, shadow patterns, and dust particles in the air. Your brains are trying to make sense of visual input in an ambiguous environment. Add in the expectation of something spooky happening and you get shared pareidolia.

That said, shared experiences are worth taking seriously. The fact that you both saw it at the same time is significant. Did you check the building properly? Any obvious explanations like mannequins, reflections, another person?

Have you got photos? Those could tell you a lot. Sometimes things that look weird to the eye show up differently on camera, which points to optical illusion. Or they might capture something you missed in the moment.

Could've been another urban explorer in the building who you didn't realize was there. Someone in costume or just wearing old clothes? That would explain the Victorian dress look and why they disappeared - they were probably in a different room.

I've had similar experiences in old buildings. I think there's genuinely something about derelict places that affects perception. Whether that's paranormal or just environmental psychology, I don't know. But your experience is worth documenting either way.