There are also fuel vents at the ends of the wings that could be venting fuel, and in the winter de-icing fluid would be dripping down.
Walking under the wing also depends on how good the staff watching the passengers are too. I was once watching loading at one side of the a/c, and happend to look to the other side to see a pair of legs standing by the engine, I ran around to find a passenger with his whole upper body leaning into the engine trying to light a cigarette !!!.
The customer services girl that should have been watching was too busy gazing around into space to notice, and she never bothered to turn around to see what was going on behind her.
Most Airports outside of the UK don't bother to enforce the rules though.
At LCA, they were letting people walk anywhere when I was there last week.
