Rule 20(2) for starters. I think there is something in the ANO as well but don't have time to look at the mo.
Bentine's son crash on 28 Aug 1971 but wasn't found until 31 Oct that year. The acft was in the tops of some tress near Petersfield, Hampshire, and by October the leaves had started to fall.
Going back to the subject, you occasionally get the feeling something isn't quite right, a 'feeling in the water'. An aircraft isn't 'officially' overdue but something doesn't feel right and so a few enquiries are made. I sure it isn't done often or lighly. We could just sit there are say 'Sod it, it's nothing to do with me' but some of us care.