Sir and Colonel and Rev, and Prof are all prefixes and should properly go in that field. However because the prefix field is generally not displayed, many people are including the prefix within FNAB, thereby triggering an error 723 which implicitly says maybe it should be in suffix. This possible transfer to either prefix or suffix is only correct in the case of Elder, which would be a suffix where it is used as a distinguisher between the elder and the younger (in terms of age) EG [(Orr-2372|William Orr the Elder], [Nairn-234|Nairn of Drumkilbo, the elder] or [Pitt-473|William Pitt the elder]) and a prefix when it is a religious "rank" (EG [Phillips-19776|Elder Phillips]). Sometimes these prefixes are put in Nicknames but this field also is rarely displayed and therefore has no use in distinguishing people. As Wikitree is now set up, the displaynames are absurd.
I would add that the suffixes KG KT Bt KBE GCMG etc all entitle the bearer to the prefix Sir and it is a solecism of the worst sort to display them without it.