John Wesley on Numbers 4:13

The altar - Hence we may conclude, that they did offer sacrifices at other times, though not so constantly and diligently, as they did in Canaan. Moreover the taking away of the ashes only doth sufficiently imply that the fire was preserved, which as it came down from heaven, Lev 9:24. So it was by God's command to be continually fed, and kept burning, and therefore doubtless was put into some vessel, which might be either fastened to the altar and put within this covering, or carried by some person appointed thereunto.