Assuming you've never had it topped off before, it
sounds like a very slow leak. You have about 9qts in the system, about 4qts of those in the pans and rest in the torque converter and cooling system. I would clean the area at the bottom of the bell housing and check for color changes there.
I'm still a little confused as to whether BMW "thought" it was leaking in the front or they "knew" it was leaking from the front seal?
I would clean the areas at the rear of the tranny as well as the side accumulator cover (on passenger side) to check for leaking as well.
As for driving it, I probably would and keep an eye on the amount it leaks especially if you park it at work during the day, etc. However, I accept no liability that it will not cause you any further issues versus letting it sit parked. Just wanted to add that disclaimer....