It's now possible, should you choose to do so, to turn off the alertness skill
so that it won't be checked when a monster tries to ambush you. Skill checks
elsewhere than the aforementioned situation will not be affected. Simply use
the skill to turn it on/off.

Rgds, your friendly neighbourhood Sage
PS. Active at boot
PPS. If you do nothing, everything will be as it has always been. So the
default is 'alertness on'.