Found in a portable toilet or port-o-let at a construction site. It is the lid that closes the toilet which is believed to be a tunnel that leads to Mexico that many of the workers at the construction site have emerged from.
The small folded opening which is found in men's Y-fronts which enables the wearer to urinate without pulling down his underwear.
"Sorry about that vicar. I believe the explosion was caused by your candle igniting a small pocket of fart which had become temporarily trapped in the piss hatch of my trollies..."(etc)