Right so I've never been to Hargrove Mill personally but shadow figures in old industrial buildings are something I've looked into quite a bit over the years. The residual energy in those places can be enormous, all that machinery running for decades, accidents, people spending half their lives in miserable conditions. It builds up.
What exactly did you see? Was it a distinct shape or more of a peripheral thing that vanished when you looked directly at it? That distinction matters a lot. The peripheral ones are often written off as eye tricks but I'm not so sure about that anymore, not after what I saw in a derelict mill outside Keighley back in 2019. Solid looking figure, stood completely still, gone in about two seconds flat.
Anyone else from Yorkshire who's visited Hargrove please chime in. Would be good to get multiple accounts and see if theres any consistency in where it appears or what time of day.