On their site they say:
Ever wonder what the path of a Roomba looks like as it cleans your house? It's an amazing combination of randomness and precision, as shown by this long-exposure shot.
The shot was taken by shutting off all the lights in the room for 30 minutes and taking a long exposure of the path the Roomba took while cleaning up. It's beautiful and surprising. I always assumed it worked with using some sort of grid, but that clearly isn't the case at all.