I am no expert on pittas, but is it really the case that there no documented records of Fairy Pitta in Sabah? (as stated by Lambert 1996). Mann's Bornean checklist states for Sabah that there are four localities, of which one, Mawau, is based on a specimen "thought to be this species". More crucially, another locality, Lumbidan, is mentioned by Sharpe (1879) (see here
http://www.biodiversitylibrary.org/item/35127#page/306/mode/1up for relevant page). Here,
P. nympha is listed under one of its synonyms,
P. oreas (
http://www.biodiversitylibrary.org/page/14501101#page/344/mode/1up), and Sharpe specifically mentions having compared a specimen sent by Low to the then BM(NH) that he compared with the type of
P. oreas (from Taiwan, current BMNH 1898.11.10.380). None of this takes anything away from the significance of the current sighting, but unless the ID of these specimens has been or can be disproved, then it means there are documented records from Sabah, just not recent ones. Maybe there is something in Mees's paper in 1977 that I've missed?