Raising the alarm in the Roman camp

[M. Demetrius]

Wouldn't the trumpeter always be ready at all times to give the signal?

Maybe I'm jumping too far to a conclusion, but it sounded like the implication was made that he would have to go somewhere and get his horn. I suggest that if that were his responsibility, he would simply pick up the horn and blow when he was signaled. Odds are, some of the wall guards would have a signal to alert the central trumpeter, and it makes sense (though I have no historical proofs) that the signal would give the direction of the attack, as well.