The hose in the pic is a water drain line from either the gas filler or the trunk seal. I don't think it was the exhaust because it is on the other side of the car. If the fluid is gas, it should evaporate after a day or so. If the stain stays the same then it's not gas. The only other fluid would be brake fluid. If the wheel brake was leaking when you parked the car it could have made the stain. Look at the inside of the tire to see if there's any fluid on it. If so then it's a brake caliper seal leaking. I would start up the car and look underneath for any sign of dripping in the area especially in the area of the fuel pump/filter around the hose bolts. They're just to the left of the hose in your pic. Also possible the stain was there before you parked and you didn't notice it.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K