If he had no intention of coming back, then he wouldn't have bothered to leave a note. But it is worrisome, with Scotty and McCoy leaving so close to one another. If there's an emergency, then he would need to be there.

If he was planning on leaving forever, he would have told us to come with him. He's an idiot, but he's not stupid enough to go on without me and Spock on that bridge.