I'm not convinced Howard was dragged across time as well as space, although I understand why people think that, as I does make you wonder what the rest of his crew were. I don't take that to mean he has been a hologram for three million years though. He may have been switched on at a later date, much like his brother was. So it's quite possible he has seen other 4000 series droids in that time period. Even if he has been dragged through time, I don't find it much of a stretch that he would recognise Rimmer as a mechanoid though. That funny shaped head and big metal body is a bit of a give-away, and as the choice seems to be between 'android/mechanoid' and 'simulant' and as he has no biological parts (apart from a bit of his brain, which we don't see) he would fit in the former category. There is a lot of untold history concerning Howard Rimmer and Sim Crawford. While I think it's a shame we don't learn more, I can see why it was left out. It wouldn't really affect the plot of this episode.