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.