A bugle is a tonal musical instrument. It can be used to play the passtune to enter the castle. Improvising with it will awaken, anger, and unparalyze nearby soldiers and watchmen, and will abuse your wisdom.Vault guards are not subject to this effect.
Sergeants, lieutenants, and captains have a 1/3 chance of being generated with a bugle. They can and will play the instrument, causing the same effects as when you improvise with it. This is not a significant issue except in rare cases, for instance if you were relying on a wand or spell of sleep to help you clear out a barracks.
'I read you by your bugle horn
And by your palfrey good,
I read you for a Ranger sworn
To keep the King's green-wood.'
'A Ranger, Lady, winds his horn,
And 'tis at peep of light;
His blast is heard at merry morn,
And mine at dead of night.'