A 'note' is a 'known mark' — from Latin 'notus' (known), from the same root as 'know' and 'cognition.'
A brief record of facts or ideas written down as an aid to memory; a short informal letter; a single tone of definite pitch; to pay attention to or remark upon.
From Old French note, from Latin nota (a mark, sign, letter, musical note, brand, annotation), derived from nōtus (known, recognised), the past participle of nōscere (to come to know), itself from the archaic gnōscere. The base is Proto-Indo-European *ǵneh₃- (to know). A nota was literally a known mark — a sign or symbol that, once recognised, conveys information. Roman shorthand scribes called notāriī used notae to record proceedings at speed, giving English notary. The musical sense — a single pitch — arrived via medieval Latin usage of nota for a pitch
Latin 'nota' was used by Roman censors to mark the names of citizens guilty of misconduct — a 'nota censōria' was a mark of official disgrace. This punitive marking is the origin of the English phrases 'of note' (worthy of being marked) and 'of ill note' (marked negatively). The same root produced 'notorious' — someone marked by widespread knowledge of their misdeeds.