Re: I don't get it...
> I'd have thought any competent sailor would have immediately thought of AIS,
They may not have it, or not switch it on, or send only basic info (position, course, speed, and the vessel's "licence plate").
> though I don't know what flag of convenience Sea Shepherd's flotilla is registered under or what its rules are on keeping ADS-B turned on.
ADS is an airborne system, thought loosely inspired by AIS. The rules are set, IIRC, by the IMO so do not depend on which register you're on.
> Even then, if you're trying to stay out of visual range, civilian maritime radars are plenty good enough to keep you over the horizon.
More to the point, marine radars work out the targets' velocity vectors and (if fed with your own vector from a GPS) calculate and display something called the CPA (closest point of approach). They get all whingy if the CPA goes below a certain threshold, and it tends to be pretty fucking obvious when someone is actively aiming at you because a) they will ignore your calls and signals and b) the CPA will stay close to zero even as you take evasive action, unless your vessel is faster than the attacker.
> Or, for that matter, a straightforward radar receiver to detect another ship's emissions long before you're in radar return range.
That doesn't tell you much apart from the fact that there is another vessel somewhere out there, which of course there will be (the ocean is far from being a lonely place), and a general direction.
Biting the hand that feeds IT
