leaking transmission oil

[rabside]

hey guys i need some help.

picked up z3 coupe 1999 2.8ltr auto last week, pre purchase check picked up some small problems....thermostat fault on pc, cam position sensor, some cracked bushes etc.

had all this work completed saturday, plus a tasty hand built air intake, heat shields etc.

was stuck in traffic yesterday for 3 hours 30 degrees c, then when cruising on highway at 120km noticed some smoke out the back, pulled over and checked under car to find it coated in transmission fluid from the rear sump pan back.

recovered car to garage, where had work done ( friend is linked to garage )
my friend is bmw freak so when he arrived we cleaned all oil from area and could find no leak. after several test drives still no leak. my gearbox is one that needs no top up ...lifetime fill.no dipstick.so we checked level using upper drain point as this should be filled at this plug until it leaks.it appeared to be maybe 1/2 litre overfilled.

so we drained and put in new bmw fluid tightened and no leak so far.

we are assuming that temp increased, oil expanded, and was released through vent on top of transmission, over the exhaust causing the smoke.

sorry if this is a bit of a story but im no mechanic, but learning so quick with this car.

any advise or similar history much appreciated.