I know of only one way that this can happen in windows, and has nothing to do with Steam other than their not-so-great decision to install all content into Program Files.
If you are on Windows 7 or Windows Vista, and you have UAC set to ON, then change it later to OFF, this can happen.
UAC does thing evil thing where if a program tries to write to a "protected" area like Program Files, EVEN IF YOU ARE ADMINISTRATOR, when you get a UAC prompt and give permission, it actually writes any files that would normally go into that directory into a "virtualized" directory somewhere else.
If you turn off UAC, that will no longer happen, and as an awesome bonus, it will no longer know about the files that it virtualized.
So in your case, if you had UAC on, installed a ton of games, then later got sick of UAC and turned it off, this would happen as you described.
See the second paragraph in "Features" in the article on User Account Control.
If you have done this and turn on UAC again, your content will come back, but be a TOTAL mess because if you've downloaded more stuff with UAC off, then that will be invisible when you turn UAC on, and vice versa.
this drove me a little crazy once before I realized what was happening.
i'm curious to know if this is your issue.
Best Answer
There's no way to tell from just the client or the SteamDB trick, since those aren't really helpful in cases where you don't download all the updates, but just some of them. Skip the first part if you just want to get to the answer itself.
Problems with the solutions of SteamDB and Steam
SteamDB will show every update of all the games you have. It also doesn't matter whether you downloaded the update or not. This list will probably have Dota2 somewhere in there, if you ever played it. So, this isn't very helpful. I don't want to see all that, just the stuff I downloaded... Relevant picture:
Again the Steam's solution is not perfect either, it's better since you can see 20 of the latest updates instead of 10, but if the game didn't publish the update as news, then it doesn't get into this page. Sure, I get a nice list of games but it's not nearly complete and there's stuff I don't want to see there, like the things I didn't download. Relevant picture:
Now, the only way I've found that works for me
I just navigate into the Steam's library folder. You can also right click a game and go to Properties -> Local Files -> Browse Local Files then go back one level to the common folder of Steam. If you have more than one library, you will need to look at each of them if you want to see all the games, but that's only to make sure you didn't miss anything. Your path will probably look like
X:\SteamLibrary\steamapps\common
for extra libraries orX:\path-to-steam\Steam\steamapps\common
for the main library that Steam keeps inside its folder.After you got into the folder where all your games are, you just need to sort by latest modified, and "Voila!", you have a whole list of updates that you downloaded in chronological order of the download date instead of the update's date (again, not perfect, but it works way better than the others):