The App Store may not record exactly what everything it does is, but you can try looking a couple places:
/Library/Receipts/InstallHistory.plist has a list of program installation history for the computer (most recent installation at the bottom, with the highest numbered entry).
/Library/Preferences/com.apple.SoftwareUpdate.plist will give you the number of pending updates as well as the last time the App Store checked for updates.
You could also try running
/usr/sbin/softwareupdate --dump-state, which dumps the current state of the update daemon to
(As a side note, in my experience XCode updates are usually over 1 GB in size, so an XCode update would definitely run for an extended period and download lots of data.)