When this time comes, the man to whom the first watch fell by lot makes his rounds accompanied by some friends as witnesses.
He visits the posts mentioned in his orders, not only those near the vallum and the gates, but the pickets also of the infantry maniples and cavalry squadrons.
If he finds the guards of the first watch awake he receives their , but if he finds that anyone is asleep or has left his post, he calls those with him to witness the fact, and proceeds on his rounds.
Those who go the rounds in the succeeding watches act in a similar manner.
As I said, the charge of sounding a bugle at the beginning of each watch, so that those going the rounds may visit the different stations at the right time, falls on the centurions of the first maniple of the triarii in each legion, who take it by turns for a day.
Each of the men who have gone the rounds brings back the tesserae at daybreak to the tribune. If they deliver them all they are suffered to depart without question;
but if one of them delivers fewer than the number of stations visited, they find out from examining the signs on the tesserae which station is missing,
and on ascertaining this the tribune calls the centurion of the maniple and he brings before him the men who were on picket duty, and they are confronted with the patrol.
If the fault is that of the picket, the patrol makes matters clear at once by calling the men who had accompanied him, for he is bound to do this; but if nothing of the kind has happened, the fault rests on him.
Walbank Commentary