You can add yet another level by having
-civilian units (controlled by the referee),
-friendly units (like from a neighboring unit, not in your chain of command, controlled by the referee),
-false units (i.e., a hostile unit pretending to be civilian or friendly), and
-more granular (flowcharted) sentry spotting effects.
For example, the sentry spotting effects could be something like this:
1. Looks like nothing is in the sentry's sector.
Result: Any unit actually there is not spotted and can move using hidden movement.
2. Something is in the sentry's sector, but the sentry can't identify it.
Result: Sentry makes a morale/reaction check. This could result in: running away; sounding the alarm; freezing for the rest of the turn; engaging the oncoming unit; or doing what a sentry normally does, shouting the challenge: "Halt! Who goes there?"
Response to challenge: Any unit may respond correctly or incorrectly to the challenge, but of course a friendly unit is more likely to respond correctly than a hostile unit, and civilian units are wild card. Did the civil affairs officer tell the locals how not to get shot by accident? Work that out beforehand, maybe with a reaction table.
Sentry's response to incorrect or no response to challenge: morale/reaction check.
Of course, there may not actually be a unit there, and the sentry is just challenging a moose that stepped on a branch. That's probably going to lead to your entire command mobilizing to neutralize the moose threat.
(Hey, don't make fun of the moose threat. I once saw a tour bus in Finnish Lapland that a moose had wrecked by letting the bus hit it. The moose was kind of banged up, too, but the bus was a complete write-off.)
The situations where the sentry spots a unit and identifies it (correctly or not) are left as an exercise for the reader:
3. The sentry spots and identifies (correctly or not) a unit as a civilian unit.
4. The sentry spots and identifies (correctly or not) a unit as a friendly unit.
5. The sentry spots and identifies (correctly or not) a unit as a hostile unit.
Note that in every case, the sentry might be wrong. There might not be a unit there at all, or the unit might actually be something else, such as a false unit, i.e, a hostile unit posing as civilians or friendlies, or even a friendly unit mistaken for enemy. What happens then should be worked out in advance on a reaction table. ("Oh, blast, I rolled a 1 and the sentry thought the wagon was full of rebels and bayoneted two sheep and a keg of ale.")
The intruder force might send a single soldier in civilian clothes ahead of the main force to distract or eliminate the sentry, especially if it knows the correct civilian response to the challenge of the evening.