When I was 8/9 years old I lived on the airfield at RAF Aldergrove. One of my pastimes with a friend was to crawl over the aircraft in the dump on the eastern side. Amongst the Lancasters, Halifaxs and Spitfires was a Sunderland. The floats were off and we were thinking of ways to convert them into boats but decided against it because it would have been too difficult to get them to Lough Neagh.
I can only think that this Sunderland must have landed at Aldergrove. Even if there was a flying boat base in the Lough it would have been impractical, if not impossible to get it to Aldlergrove there being small roads flanked by trees.
There was no apparent damage to it as far as I can remember so maybe it just got lost and used the BABS at Aldergrove just as it ran out of fuel.