He had to pack up things - a task which might take most people less than an hour took Poseidon half a day because he couldn't decide what he needed to bring and what should just stay here, and when he was done deciding he couldn't remember half the things he had put inside the bag already, and then he had to drop his bag off at the counter and pretend he wasn't Greek or god of the seas and be Michael whatever, only to be manhandled onto a flying metal box which was jiggling in the air like it could break in half at any moment and crash in a huge ball of fire and kill everyone on board.
And that was before he even landed on the other side.
But sometimes he had to go. Sometimes Zeus was being too much of an asshole or Hades was being stupid, or the others were being reckless and fueling invisible fires, and Poseidon ended up going there to calm the storms. He had his own grudges and disputes, and there were some things no one could back him down from, but for some reason he often found himself caught between two crazy Greeks, especially since the fall of Olympus. It wasn't a nice place to be, but sometimes the earthshaker had to be a peacemaker.
This time though, he was on his own. He felt somewhat betrayed by Zeus, and this in turn has led him to distancing himself from everyone else.
He made a last-minute trip out to his halfway-constructed house, making sure everything was in place before he left. Obviously he wanted to get his project finished but he couldn't do that with the number of trips he was making off the island. In the meantime he had to preserve what he had up there well enough so that it could weather a few storms without deterioriating so much that he had to start over again.
When everything was sorted and prepared - at least in his mind - Poseidon returned to pick up his things. He wandered into the kitchen and got himself an apple juice, unaware that there was anyone else here, and glanced around for anything else he might have left behind in his absentmindedness.