since it's mentioned, the HL2 source code is original. it has been compiled, and yes, the person who did is vaguely known. not enough to be caught, however. he/she/it installed one of those customised build of key-type recorder onto Valve's computers.
back to GTA3, you can't help those little AI bugs. hey, all the fun
it's not a 'realistic' game anyway.
[edit] oh yeah, did i mention all of the code was stolen, just that only parts of them are spread around?