Ha. That's funny. I think that's more of a reused assets question than an easter egg. I have also heard that in movies where the later movie clearly used the same sound effects package that they bought from explosions r' us.

When the witchers are drinking and Lambert puts on Vesimer's black stetson he looks like the bounty hunter that actor Lee Van Cleef portrayed in the "spaghetti westerns" : "The Good, the Bad and the Ugly" and "For a Few Dollar More"

In Novigrad's fish market a dude was singing "I sing in the rain... just sing in the rain" and it was actually raining.

Overheard a strumpet outside The Golden Sturgeon quoting Madonna: It's a material world, and I am a material girl.

a noodle-shop owner/sword-smith called Hattori, is a clear reference to Hattori Hanzo from the Kill Bill movies

Overheard two people arguing and the tail end of it was a woman saying to another "I'll stick a rake in your cunt!"