I put them on top of the model of the building, with a die indicating which level/floor the mech is standing in. (I'm an American, where the 1st floor is the one on ground level, so no need to worry about G then 1 for the 2nd story, so on)
Of course we also use dice to show TMMs/who's already gone, so a different color for "what story in which the unit is located" is a good idea.