I was cleaning up my Mac (on El Capitan 10.11.2), and I tried to delete some obscure old Word file. When I clicked on "empty trash", it gave me the message that the file was being used. I didn't know what was using it, and didn't want to cause problems, so I used
lsof | grep /Users/myusername/.Trash and found that the QuickLook process had it open 12 times! Should I delete the file? Why on earth does Quick Look have it open at all?
PS: I quick-looked the file again and ran
lsof | grep -c /Users/myusername/.Trash, which told me that it was then open 18 times, in case that helps.