[cinntaile]

The original Half Life engine (goldSrc) was based on the Quake engine, on Wikipedia it is mentioned that it was heavily modified. Half Life 2 used the Source engine and it doesn't necessarily mean that a lot of Quake engine artifacts are left, no?

[beebeepka]

It doesn't have to be a lot, though. After all, Valve was hacked like an year (sorry but it's been literally 20 yeas) before HL2 was released and yes, some Quake code was identified immediately.