[El Gato]

With modern technology, shouldn’t the whereabouts of this ship be easily determined?

Sure, I'm sure the French and US Navies know exactly where it is. Doesn't mean they are telling. Let the pirates worry about if those Navies have accurate information or not.

[fso301]

a small French warship, the Commandant Bouan, maintained permanent surveillance of the vessel.

So, it seems as if the whereabouts is known.