I haven't been inside a 4L85-E tranny (yet), but practically all Hydramatics have a flat, square or flat, round magnet in the pan, not on the drain plug. I just yesterday replaced the filter and drain plug on a 4L60-E in an '01 Silverado. No magnet on the plug, but there was the ever present magnet in the pan.
It's been a while since I changed the oil in my 8100, so I don't remember if the oil drain plug is magnetic.
Jim & Linda
'03 Winnebago Adventurer 35U (W22)
'06 Saturn Vue Toad