Hi,
I see this before, and yes, it is the low ink level message causing the printer not going in to sleep mode.
This is to warning you that there might no be enough ink on the printer for your next prints, copies or faxes.
I agree that is not nice to have the printer on all the time, but definetly worse to run out of ink on the middle of a work that you have to have ready in the next hr. :)
Best regards
Mike G