I think you have to answer that yourself. As in, I don't think there is a
right answer.
Since it's non-invasive (that's important to me) and more or less risk free (I mean, ok, they might roll off the table, but....), I don't worry about consent. I mean, I don't specifically get consent to do my PE, either. So if I found something, I'd just go back to the client and say "Hey, just to be extra cautious, I took a quick glance with the ultrasound, and I saw blah blah blah." I wasn't charging clients for the FAST unless it was a fully-indicated diagnostic in my first 4-5 months of practice since I was doing it as much for me as them and it wasn't strictly 'indicated'. I was charging for obviously-indicated patients (the 7-year-old lab with a fluid wave and white gums.....).
But if that makes you uncomfortable (the idea that you're scanning without consent) then you shouldn't do it. In that case, you would just tell the client "Hey, I'm trying to get some more experience with our ultrasound equipment - do you mind if I do a quick scan on Fluffy? I'm not going to charge you for it" or some variation of that that best works for you.
A good example would be a parvo patient we had that had been in hospital 48 hours by the time I picked it up. When I got it, the patient was on its second rebound (so, it had already had the first fake bounce, gone back downhill, and was now improving for reals) and doing great and I was discharging. It was a quiet morning, so when the techs got it out of isolation to send home, I just scanned it quick. Purely for experience.
Found a big (really impressively big) structure with an anechoic interior right over the cranial abdomen. It looked exactly like a pancreatic cyst, except those aren't documented well (or at all) in pediatric patients (at least, that I'm aware of). And I couldn't confidently associate it with the pancreas. Puppy is recovered, bouncing around, eating - clinically great.
So I just told the owners "Hey. I'm trying to get more comfortable with our ultrasound, so I was scanning your dog and I found this weird thing and I don't know what it is, etc. I know fluffy is doing great, but you may want to follow up with your doctor or a specialist." Super nice people - they rushed off to IntMed who said "wow, that's weird". Talked about cytology, surgery, or just monitor. In the end they did nothing, rechecked in a month, and it was gone.
So that's how I handled an incidental finding that from MY perspective needed intervention (in the sense of "somebody smarter than me should check this out"), even though in reality it turned out not to need it. Downside: I cost the owners $500 in an ultrasound and recheck. So yes, there's a risk if you find something incidental that leads to an unnecessary work-up. Thing is, you never know how necessary it is until after-the-fact.