AS the Canberra sails away this evening on the outward journey to England, on board will be a long-standing Hongkong couple departing these shores whose final memory of the territory might well be less than happy. Having no children of their own, this respected couple (he was high up in the judiciary) treated their pet dog almost like a surrogate child. And, quite naturally, anxious to take the family pooch back with them, she took the dog along to Kai Tak to be air-freighted to England to undergo quarantine while they made their leisurely way home. A straight-forward matter, one would have thought. But not to the customs officer, it wasn't. ''Very old dog,'' remarked the customs man as he cast his eye suspiciously over the dog. She, of course, concurred. But his next remark almost floored her. ''Ah, smuggling dog,'' he retorted, before putting on surgical gloves and thrusting his hand into every orifice of the hapless pet looking for non-existent drugs. The lady, we are told, had to call for a chair and a glass of water to compose herself. Happy to report, though, that the dog is now in England awaiting a happy reunion with mum and dad.