The notes section can be a super-useful too, and I also use it when I have difficulty tagging/deciding the warnings.

And you've touched on a point glitteryv brings up above, too, that tags are used by readers for any number of ways, not just as a warning system, but in a way of helping determine what they want to read. I think I would still probably put anything about noncon/dubcon in the tags myself just because I know even mentions of it can trigger someone. I would still add in the notes that it's offscreen, but I think I tend to err on the side of safety. (And to be fair, I don't write much noncon, but when I do, it's front and center - I don't put noncon offscreen. Usually either I can leave it out altogether or it needs to be highlighted in some way. Unless it's just a part of canon that I'm acknowledging, which... ugh, that gets messy.)