Looked through the photos a couple of times now. There's definitely something in frame three - that dark mass near the doorway on the left. Hard to dismiss it outright.
That said, I want to play devil's advocate before everyone goes full ". Definitive proof." A few things worth checking first:
Camera settings - what ISO were you shooting at? High ISO in low light creates noise that can cluster into shapes our brains want to see as figures, Lens flare/internal reflection - old asylums with broken windows are notorious for weird light behaviour, Motion blur - if someone moved through the shot during a long exposure, you get exactly that kind of smeared shadow shape
I do a fair bit of ghost hunting around Somerset and I've been fooled by all three of those more than once, genuinely embarrassing stuff.
But - and it's a decent but - frame three does look different to the others. The edges seem too defined for noise, and there's no obvious light source that would cause reflection from that angle. Worth ruling out pareidolia first though.
Anyone run it through Lightroom or even just cranked the exposure/shadows right up? That usually reveals whether there's detail hiding in the dark mass or if it's just a flat void. Flat void is actually more interesting in my experience.
Would love to see the RAW file if you've got it. What camera were you using?