Looks like they've optimized it fairly well to prevent battery drain: https://www.xda-developers.com/how-google-pixel-2-now-playin...

Personally I've only ever seen "Pixel Ambient Services" show up on my battery list once, and that was 1% usage after a fairly long day out.

Theres a whole bunch of crap Android phones do now in thr background like the "Ok/Hey Google" assistant voice shortcuts. I turn it all off. But supposedly it only uses a special lower power chip to do these passive listening actions. Not sure about music recognization though -- seems like it would involve a decent amount of memory even if performing a convolution..depends how much buffer time.