Thermostat OEM Audi or Mahle

I finally got around to installing a new thermostat/o-ring/housing and coolant reservoir/cap yesterday. When removing the thermostat, I noticed that the bottom bolt was VERY loose... I had done the thermostat without housing about a year ago now, had never leaked coolant before... My best guess is that I just didn't torque it enough and it slowly loosened, but only when cold (just a guess, but would help explain why no drips when car was running on ramps) and in small enough amounts that I did not notice a puddle on the ground.

I had a 96 and have done that job twice, make sure replace the thermostat otherwise it's a do-over several months later because you tried to save $20. Been there etc!! Change the belt, tensioner, water pump and thermostat.