Maybe the most perplexing situation yet on my 2011 iMac running El Cap. I ran Time Machine around noon today for the daily backup. I used the computer a fair bit today, but only for email, Excel file work and some Squarespace website updates. I manage email well, and I make sure the delete trash is emptied regularly. No big downloads, no unZIPped files, no video/audio. I fired up TM 12 hours later, and it said preparing to backup 13.xx GB. Huh? Where could that have come from? As I mentioned, no new downloads or installations of any kind. The only unusual move I made today was to activate TRIM on my most recently installed 1 TB SSD boot drive. Any ideas about the 13 GB? I scanned "all my files" real quick in Finder, in chronological order, and I only saw a few MB worth of new files today.