Meanwhile, at the Carl Sagan Memorial Station (née Pathfinder) on Mars:

Unlike Spirit and Opportunity, Sojourner could not directly communicate with Earth and had to rely on the lander to transmit messages about its health and travels. So when contact with the lander, which was designed to last one month, was lost after three months, ground controllers were not sure what became of Sojourner.

Now, it seems the rover kept moving, apparently trying to reach its companion. "The rover was programmed so that if it didn't get commands from the lander, it would assume it somehow got out of radio contact behind a ridge or rock," Parker told New Scientist. "So it would drive to the lander as best it could and keep trying to re-establish contact", circling it as it got close, he says.

When it was last seen, the rover was 13 metres away from the Pathfinder lander. Now, it appears to be about 6 metres away, according to the new MRO images. "I think the simplest explanation is it started to drive back, got about half way, and stopped for whatever reason - it may have thought it got there," says Parker. "The other possibility is it drove around and around for who knows how long and simply failed at that location."

Emphasis mine. Full article at New Scientist.