He told everyone he was going on a long vacation with his son, not into hiding and completely off the grid. You expect to be able to get a hold of someone on vacation eventually, email, phone, mail forwarding, something.
After the hurricane when Deb was discovered to be missing, they would obviously have tried to contact Dexter, and when they couldn't, that would be pretty surprising to them. Find the wreckage, and its "ohhhhh, he's dead, that's why he hasn't responded to our messages that his sister is missing." I think it's a pretty natural train of thought.
if there's a plothole there - it's not concluding Dexter is dead - its, why doesn't anyone care about what happened to Harrison? Batista just sounded sad for dexter dying, as if he understood why he did it and just wished he could have talked him out of it. No regard for the fate of Harrison whatsoever.