Here in Ireland, it would be the disappearance of Philip Cairns.
In 1986, A 13 year old school boy left home to return to school after lunch, and joined a throng of perhaps 1200 other school kids also returning from lunch. He vanished, without a single trace. All the schools involved had school uniforms :- somebody somewhere should surely have noticed a kid in uniform getting into a car, or even walking the wrong way (away from school).
6 days after his disappearance, his school bag was found in a lane about 200 meters from his home, with the religious knowledge and geography books missing. The lane had been previously searched, and the police believe the bag was placed there shortly before it was found.
Nothing more is publicly known than what I’ve recounted. About 3 years later, the missing school books were found on enclosed waste land about a mile from Philip’s home. This has never been publicly disclosed, for exclusionary reasons (I know this because a then girlfriend worked in a office building adjoining the waste land).
As it happens, my childhood home was 10 houses away from Philips’s : I spent many weekends at the time as part of search parties combing the Wicklow mountains.