HL2 also had a 700MB update yesterday with no official update notes released so I'm a little more suspicious on this one admittedly. though my friend explained in that case that it may be some large update to Source engine in general since other games use HL2's resources to run
That was considered part of the evidence. In fact the update was reported by some to be 3.7 GB or in some extreme cases, 7.3 GB. Which is especially odd, those numbers. There were some discrepancies though. People had 500 or even 50MB updates. I myself didn't get an update at all.
What really irked me though is half my achievements are gone.