At the start of the game, the defending forces are not expecting combat. Instead, they are resting, eating, servicing equipment, talking about the girls/bug-eyed monsters back home and so on. Because of this, only the sentries will be active to start with. Until the alarm is raised, sentries move as described above and none of the defender's other units may move or fire. The alarm can be raised in several ways, listed below.
Spotting Distance
A sentry will spot any enemy model that is within their spotting distance at the end of any turn. This distance is equal to their Initiative characteristic in inches. For example, a sentry with an Initiative value of 4 would spot any attackers within 4" of it, regardless of whether the attacker is in cover or not.
Vehicles
If the attacker deploys any vehicles or bikes on the table, then the sentries will spot them at the end of the turn. Tanks, Dreadnoughts, armoured personnel carriers, etc are all too noisy to avoid detection. This does not apply to vehicles kept in reserve, unless they enter the board in the attacker's subsequent turns.
Weapons Fire
The alarm is raised if the attacker fires any weapons except sniper rifles and needle weapons. If a sentry survives being hit by a sniper rifle, they raise the alarm.
Close Combat
A sentry that is attacked in close combat will raise the alarm if they survive to the end of the turn. If the sentry is killed in close combat, then the noise of the fighting may raise the alarm anyway. Roll a D6, on a 4+ the fighting was heard, or the sentry screamed out and the alarm is raised. A unit which kills a sentry in close combat cannot make a sweeping advance - because they're sneaking up, they can only consolidate.
Bodies
If a sentry is killed do not remove the model. Instead, leave it lying in position. If another sentry approaches within their Initiative distance of a dead sentry, then they will find the body and raise the alarm.
Note that all the other defending models count as sentries for purposes of sounding the alarm, they just can't move.
After raising the alarm all sentries are removed; they go to ground, run away or move to rejoin their units and take no further part in the game. The defender then takes a turn immediately, moving and fighting as normal with their remaining forces.