It was remarkably hard to get any privacy on the Prisoner of Azkaban set.
Trailers were out, since the kiddies had an unfortunate habit of barging in and demanding autographs for their friends and family.
Most other places were a public thoroughfare, and therefore were pretty much a no-no.
Gary and David thought they'd found the perfect place: an empty walk-in cupboard. It obviously wasn't being used for anything at that time, so it was unlikely that they'd be disturbed in there.
It was, therefore, quite a shock to discover that Alan and Jason had, seemingly coincidentally, had the same idea.