With the Xena disk locks (such as the XN15), I can't see any reason why someone who could tolerate the alarm for about 5 seconds couldn't just unscrew the alarm bit and silence that, giving them time to hack at the lock in relative peace.
I'd have thought that if an alarm went off, people would look, but if they just saw someone bent over and it then went quiet, they'd assume they'd tripped it accidentally.
Anyone got any experience of said locks? My thoughts are the alarm bit would be more for me - stop me riding off with it still in place...