I noticed my C: drive was full, it is a 120 gig partition on my SSD, I have a seperate 400 gig partition on the SSD for games (meaning if I format C: for OS reinstalation the games survive).
So thought was odd, as it turns out everything steam downloads is first copied to a downloads subfolder under the steam install path (which is probably program files(x86)/steam/steamapps). So if you one of those people who installs to hdd's to save your ssd flash writes, or are wondering why space is low, this could be why, this folder on my system had 40 gig of files, I was in the middle of downloading skyrim and also had recently updated gta5.
You can use steam spreader to have it hosted on a hdd and symlink it. As its probably a waste of valuable SSD space to use just for a download cache folder, installed games dont run from this folder.