I don't know how they'd know unless they could read your computer's process list or something like that... And even then, how would they know if a process is recording or just receiving audio?
They probably scan audio tracks uploaded to music sharing sites, a few online streaming services do this for video to identify accounts ripping the content.
If you're doing it for personal use I see nothing to worry about
I know that this is possible, but I've never actually heard of it being done on streaming sites. I've only heard of it with e.g. prerelease copies of movies sent to critics or something like that.
Crunchyroll used to (very blatantly) but I don't think they do that anymore.
No idea of any other on-demand streamers using it unfortunately, however WWE, warner bros, FIFA, formula 1 and a bunch of sports organisations with their own subscription service use this tech in a transparent way to kill IPTV streams quickly
Which might be good for detecting if somebody somewhat widely distributed a recorded copy, but not if it was just for personal use. Also, if the capture process creates loss there's a good chance it might degrade the watermark as well
Not even just the capture process. Re-encoding the track to release it into anything lossy, even at a high bitrate, is going to destroy anything that’s inaudible. It would be like trying to read the microprint on the photocopy of a photocopy of a bank note. It’s not going to happen.