Transmissions drain constantly. I've noted this with junkyard ones for example. They'll dump the fluid out of them when they are removed and put them in the warehouse. Six months later if you buy one you'll find almost a quart of fluid in the pan. If the converter wasn't drained then it's about ten times as bad. A C4 like you pictured without the dipstick in will very slowly make one heck of a mess as it drains back internally and overflows on your floor.
