Perhaps, as in some schools, that's how they check for metal fatigue? They wander around the aircraft until they see water pouring through towards the ground from the underside of the wing which will in logical turn indicate to them where the holes are on the upper surface of the wing. In Africa we do this to check for the lethal Piperus Cesnexis, a really evil metal eating bug which is much favoured by the convicts in the chain gangs for it will easily chomp its merry way through the padlock system on ankle gyves.