Assuming a linear acceleration from the time off the accident, assuming that relatively speaking Red Dwarf went from a standing start (Or at least a comparatively slow speed), assuming that when Holly set Red Dwarf on her course out of the solar system that it went in a more or less straight line with little or no manouevering and assuming that Earth stayed more or less in the same place (Current knowledge is that the galaxies are moving away from the center of the universe, so it could have been moving away from Red Dwarf, in the same direction or somewhere in between). Assuming all of this and given that Red Dwarf broke the Light Barrier in Future Echoes at that stage it would have been somewhere around 1.5 million light years from the solar system. Space is huge, Red Dwarf is supposed to be around 5 miles long IIRC which is relatively tiny. Best guess is that once the JMC/Space Corps stopped receiving post pods and updates from Red Dwarf it was very quickly written off as "Missing, presumed lost" and no further effort was made to find her. Maybe someone even received one of Holly's distress/warning calls and just reasoned that the ship was at that stage unsalvagable or not worth the effort of trying to recover and she was written off, it certainly sounds like Red Dwarf was an older ship going from the likes of Howard Rimmers comments so the cost of finding, catching, recovering and decontaminating her would probably be prohibitive compared to her value. The longer Red Dwarf was travelling for, the further out she got and the less likely it would be that another vessel would happen across her. Just as an aside to this, quite some time back in reference to another post I pointed out that going by Lister's comments in Future Echoes Red Dwarf should still be on the out-bound portion of the turn Holly initiated in Series I and should still be travelling away from the solar system at a considerable velocity, not closing the gap in the slightest. It's a relatively easy one to fan-explain to be fair, either Holly managed to sling-shot Red Dwarf around a suitably large planet or star somewhere and sometime off screen, you can maybe assume that the Nanobots were able to stop Red Dwarf dead and/or nano-Dwarf had a much smaller turning circle. We also know that the crew have traveled through wormholes in Starbug in the past, so it doesn't seem impossible that they may have used one or more of them to jump Red Dwarf much closer to the solar system than she would have been if she was just under her own power. It's just that all of this happened off-screen in between episodes/series and you can mix and match any or all of these to suit your own thinking.