Raising the alarm in the Roman camp

[M. Caecilius]

Lots of things are possible. Caesar mentions trumpets as signalling for attack, and at least one other time as a signal. I don't remember any mention of whistles, anywhere. But I'm not a historian. I just sort of hang around the edges of the circle.

Do you have a source for Caesar using them as a signal (unless it's a signal in battle itself or to initiate one when the troops are ready anyway, such as when Labienus prepares to push back the Treveri under Indutiomarus)?

Whistles would be interesting - they carry a long way - but I don't know any reference to them either. Cassius Dio (54.4.4.) says that Augustus added a bell to the statue of Jupiter Tonans, "for those who guard communities at night carry a bell, in order to be able to signal to the inhabitants whenever they need to do so". Other examples, from civilian contexts, do mention trumpets to summon the neighbourhood or the watch, such as various temples when about to be robbed by Verres (in Cicero), or the episode where the fire-fighters mistake Trimalchio's trumpet concert for an alarm and take axes to the door to mill around the feats with their buckets and mats, but in the cases of trumpets I know of, we are not talking about a single exposed sentinel who needs to have at least one hand free to survive, beside sounding the alarm. Whistles do not appear in any literary work that I know of, but I would greatly appreciate the information if anyone comes across them!