1. Shortened version of 'Signal Officer', primarily in the Army. Most often used to refer to the latter instead of his/her name and rank.
2. Can be used to refer to the Signal Officer at any level of command (e.g. Battalion Sigo, Brigade Sigo, Division Sigo).
Soldier 2: Oh, the Sigo? Nope. Haven't seen him all day.
Soldier 1: Does he EVER work?
Soldier 2: Beats me.