According to the second of the two squibs in Notes and Queries on page 336: the verb is in the subjeunctive who may live?.

According to the entry on qui vive in the Trésor de la Langue Française Informatisé, one of the earliest citations is in Thomas of Woodstock. 1419. Chronique du Religieux de St-Denys: "Interrogati secundum communem modum loquendi: Qui vivat, qui vivat? respondebant: Rex, regina et dux Burgundie, nomen Dalphini tacentes." (They were challenged in the usual manner saying: "Qui vive? Qui vive?" They answered "The King, the Queen, and the Duke of Burgundy," being quiet about the name of the Dauphin.) It's in the subjunctive there in Latin. Seems to me that on the Q.T. means quietly, whereas as being challenged with qui vive? one would speak up if one knew the password and could pronounce any shibboleths.

Ceci n'est pas un seing.