The sea is big though and even a few miles for something seemingly so unsophisticated is a big deal for target aquisition. The camera is barely above the waves the horizon must be very short so even a few miles away from where you think it is might be enough to make interception difficult. Like I say I'm amazed that the Russian ship appeared to either stationary or nearly so given the proven risk.
They probably used a variety of sources of information to find the likely location, a ship that big does give a fair signature on thermal/IR optics as well even under those conditions. IIRC with some previous incidents they used airborne drones to narrow down where the ship was.
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