Without other knowledge, I would have assumed that they left it in lunar orbit (or crashed it back into the moon like the later ascent sections of the lunar modules) rather than taking 2 extra tons out of lunar orbit with them.
My first thought exactly. They must have left the lunar lander attached to the command module for the Lunar to Earth burn, which doesn’t really make sense to me, but it’s the only explanation that I can think of.
OK, I did a little research here: http://history.nasa.gov/SP-4029/Apollo_10i_Timeline.htm
It looks like after they separated from the LM ascent stage, they fired off its engine, and it must have gone off on its own.
Arg, I just saw this sitting in a browser window from quite a while ago... I forgot to hit send. Apologies if you guys have already hashed this one out. :)