[RKDinOKC]

Oh Oh, I know I know!!!

The sewer vent for the toilets sewer lines is clogged. This tries to remove the liquid waste and vent at the same time thru the toilet to equalize the pressure in the sewer line. Because of the water trap some poop may not have gotten past the trap into to toilet line by the time you flushed just the liquid. That poop was caught up in whatever currents their were emptying water and venting the line bringing it up out of the trap and into the toilet bowl.

That or instead of either potable or gray water being used to flush the toilet it is actually using black water. I am assuming this option it would be more likely there would be poop in the toilet BEFORE peeing in it.

Either way the toilet would certainly not smell as fresh as it should.

If I don't know the right answer I can sure make something up.