Weird issue with Android that hit my wife's phone. Checking storage said she had 300 megs of 52 gigs free, but the breakdown of data only added up to about 20 gigs used. Mounted it over MTP, which sucks since you can't use WinDirStat or something similar on it. Did a nice binary search to find the offending item, which (from memory, so maybe not exact), /Android/data/com.android.google.gm/cache/*. In there were a ton of IMAP cache files taking up a whopping 31.2 gigs. com.android.google.gm is Gmail. Checking Gmail in the apps list claimed it had like 100 megs in the cache, and clearing cache from there did no good. Ended up deleting them from Windows and it doesn't appear to have hurt anything, just freed up over half of her storage.
Just throwing it out there in case anyone else suddenly finds they're struggling for space when it seems like they should be barely using any.