Ah, it's this. Ford figured they'd have this conversation sooner or later.

"There's a number of reasons," he says, matter of fact, tucking the scanner into a pocket. "What I told Luke was true. I do want to keep an eye on you, make sure you don't cause too much trouble, and find the source of the radiation. But, besides that..." Ford pauses, looking back up at Bill. "I wanted to come," he says, and there's a note in the tone that sounds almost like a shrug. "I want to find what's making you vanish and stop it."

It doesn't sound like he's lying. But it does sound pretty unbelievable.