Check the options menu. It should be in either the !Downloads folder of the games folder, or whatever you set it to.

Thx. Found it.

That gives rise to another question. Is the stuff in there still important? I found, apart from the map and soundtrack, patch files cakked setup_the_witcher3_exp1_2.0.0.42

I guess those are patches for the HoS expansion but shouldn't they get deleted after patch installation?

I ask because it's more than 4GB

You don't need the setup files to play the game. But the client may back up the downloaded files, so that should you ever want to reinstall the game (or migrate to a different computer) you can just copy the files, instead of re-downloading.

EDIT: I should clarify since I'm enough of a pedant to call myself out on this: you don't need the setup files any more, once the game is installed and playable. You certainly do need the setup files to play the game, but there's an intermediate step involved XD