File indexing is wonky, so Windows at times ends up keeping a cache or copy of everything, doubling occupied space for seemingly no reason. 100Gb gone missing for no reason, it’s usually file indexing at work.
I still can't understand why MS doesn't use the features they themselves implemented in NTFS to make the search work like in Everything. Those features are not even new anymore, and still they implement a search that sucks so hard that i'd rather search manually by browsing random folders.