[Elsa may not be queen anymore, she may never be queen. But she had been educated to be one, to be a good ruler for her kingdom. In a place where all the government fell in the shoulders of the monarch, it would have been Elsa's job to watch over the trade routes with other countries, which ones were safe, which ones were being rentable... It was a business world way above that of store clerks and workers, but it was business all the same. And if there's a language that Scrooge speaks it's that of business, the best profit at the lowest risk, though he also knew when to take high risks if the pay was exponential to it.

Knowing this, it was very easy for Elsa to know what he wanted and what he needed. And the sinister sounding chortle didn't bother her in the slightest, seeing how happy he had been of finding such a huge treasure she can understand well his wish to keep it for himself. And technically it was theirs, the way the maps worked the island was an unknown one except for them. Following the logic of colonization, the island belonged to them and so did the treasure, protecting it from others was the obvious step.]

We could also leave a fake reward at the end of some of those markings, maybe a map with instructions to another place. Something that would make them leave the island before they bothered to explore it throughly, just in case someone would be able to crack our own marks.