I came across the following passage in the Visuddhimagga (Bhikkhu Ñanamoli's Path of Purification, page 106):
And this is said in answer to the question of Sopáka: “Perception of visible forms is quite clear, Blessed One, perception of bones is not clear” (Source untraced29)
The corresponding note is:
29. Also quoted in A-a V 79 on AN 11:9. Cf. Sn 1119. A similar quotation with Sopáka is found in Vism-mhþ 334–35, see note 1 to XI.2.
It seems that there are quite a few untraced suttas and untraced commentaries buried in the various commentaries and treatises. Do you know of any website or book listing and/or analysing all these untraced suttas?