While GMing, I had to go through a description of what Senses skill is and how it applies. (PCs as caravan guards getting jumped.) Figured some of you might want to understand the way we see it. Hopefully no surprises:
Senses is typically used defensively, in other words - as a resistance. Therefore, if a sentry is actively aware, the person trying to sneak up on him should roll their attempt at being stealthy and resist it by the sentry’s (K +SEN) x 5%. Use of K x 10% + SEN x 5% really shouldn’t happen, though of course it does.
If you were to use your Senses as an active skill, you would take a combat round. Eventually, your eternal vigilance would result in you passing out, and thereafter not protecting your caravan. When would you use it? Well, if you heard a noise in the darkness and were trying to see what it came from, that would be an active Senses task. Of course, anyone using Senses constantly would on average fumble every 20 turns or about every three or four minutes
So, how do you know if the sentry is actively aware? What do you use to find out if the sentry wandered off or fell asleep? Well, asleep is different, but to determine if the sentry gets to use his or her SEN skill levels or not, I think you give him a Etiquette task but use Willpower as the base attribute. The assumption here is that disciplined sentries will have ETQ. If the sentry is undisciplined, there is a greater chance that he will not follow his orders and either be somewhere else or day dreaming. Now missing sentries cannot resist sneaking, but day dreaming ones can. Where an alert sentry resists at (K +SEN) x 5%, ones who miss their alertness roll will resist at (Kx5%) -10%. The “-10%” is for distraction. Now if for whatever reason, you want to have the two sentries arguing over which jousting champion is going to win at the tilting yard tomorrow, well, the distraction value should go way up.