Ho ho!! What an adventure for the big boy. Glad to hear that all was well and Mason was caught safely.
I recall the very first GALa that I went to, over in the Alloa equestrian centre. A greyhound got loose and it was pandemonium as the wee monkey raced round the stadium at the speed of light whilst his poor mum tried to catch him! Not that we laughed, of course. Nope, not us....
Imagine that person leaving the gate open when they know it's a dog centre!! As you say, you had already warned the staff, so at least they were vigilant and on to him right away, and it sounds like a good site as well which is reassuring with your Houdini
Hows the girl that caught him doing? she must have been frantic at the thought of getting him back before home time. They should get some 'scent of bitch in season' to tempt him back with, because now he knows the way out, they'll need eyes in the back of their heads to keep him in. What about those brake things they fit to supermarket trollies to stop them wandering? could they be adapted