As I mentioned, I suspect that this is an active choice, as just displaying the icon whenever a media context exists seems much easier than inspecting the audio stream for non-zero volume media.
I can only assume that there are legitimate reasons for this as well, e.g. websites preparing/maintaining audio context for lower latency when they intermittently play audio etc.
Someone else mentioned that cloudflare uses the webaudio for verification challenge, but only triggers it briefly.
Ugh... Seems like we need an audio API web permission, or maybe do something like browser already do for some of the other APIs and actually require API users to actually play something or display a warning/play an annoying chime otherwise.