I'm also new to Zotero (also coming from Citavi) and I just spend half an hour trying to figure out how to search for a collection until I figured it wasn't possible. I will have to read up on why this is hasn't been considered for implementation in years, but for now I'll add my vote here: this would be a very useful feature.

I vote for this feature too! This is such an indispensable feature.

I have a way that allows for quasi-subcollection searching, perhaps it helps those of you who need to search in subcollection.
When I open an important subcollection, I add an item in the subcollection with the title: "Tag:[tag name] (instead of "tag name" you can put any tag you want. Then I tag this item with a real tag.

When I search for this tag name either in title search or tag search, this item would come up. Through this item I can find the whole subcollection.
Hope that helps.

@shivams, good to know. Thanks.