Never been to Hargrove Mill specifically but shadow figures in derelict industrial buildings are incredibly common reports and I've documented a few myself over the years up here in Cumbria. Old mills especially seem to attract them, whether thats residual energy from the workers or something else entirely is the question nobody can actually answer.
What did yours look like? Solid black, no features? Did it move with purpose or just drift? The ones that genuinely unsettle me are the ones that appear to notice you, the ones that turn or react. Those are a different category in my experience compared to your standard residual haunting.
Also really annoyed they've demolished it now, these places need documenting properly before they go and once they're gone thats it, the activity almost always disperses. Would love to hear from anyone else who got in there before the wrecking crew showed up.