Scattered around the planet.

People don't know this, but Jimmy always had a hankering for space travel, was jealous of the astronauts, and volunteered for the only program which would have him. He arranged it to look like a mob hit so as to keep the program secret. After all, most people couldn't just disappear without someone suspecting something.

He forgot and stepped outside for a smoke, and kerblooey, he was reduced to cellular matter, vaporized, and spread around the planet. A little known fact which explains why he's never been found on Earth.

The small traces of methane found in the Martian atmosphere are from his last meal, an enchilada with refried beans.

I didn't believe this, but then checked Snopes and they don't deny it, so I suspect it must be true.