I find that I sometime forget that I have selected tags and am surprised when a search doesn't turn up what I'm looking for. A simple flag to indicate that tags were selected would be a useful reminder. Perhaps something as simple as coloring the filter icon, as in the image. This is necessary because sometimes the selected tag is out of site, as is always the case when applying a search with zero hits.

https://s3.amazonaws.com/zotero.org/images/forums/u493397/svpi0rhhpmhf7zoe7cjj.png