The answer is simple: there was motor oil spilled on the seat in the past. The upholstery was cleaned up, but the foam cushion inside is acting like a sponge and expressing out the residual oil through the upholstery surface every time someone sits there and compresses the foam with their body weight. It will never evaporate or wash away and will not appear unless sat upon. ( love your show ! )
So you are the mechanic who spills the oil and owns the dry cleaner next door? 
J, There’s No Need To Open A New Discussion When Adding A Comment To An Existing One.
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Thanks, CSA
Hi ~ We had the same problem and fixed it. We have a Honda Civic 2000 we got recently and it gave our friends wet butt too! The problem is: rain water is coming in the back hatch and not draining properly. If you take the interior rear paneling off, you can see where it should drain. But, stick a hose up there and you see it drains right into the trunk and pools under the spare tire. When you drive, accelerate, turn, brake… it sloshes forward and soaks into the rear seat cushions. We had so much water, it sloshed all the way to the front floor! Our solution: my husband siliconed around the lights AND DRILLED a few small holes in the interior to allow the water to drain to the ground: 1 in the raised area under the lights and one in the bottom of the trunk area. IT IS DRY!! YOUR friends butts are brown/wet because your seats are dirtier than our seats (our friends butts were wet but not so brown
Beth