Unless you invested in perimeter golems, guards are people. They have off days, get distracted, and are people. It's unlikely that every guard you meet will be completely on top of things all the time, so roll for their mood when you come up to a guard post. This doesn't replace a reaction roll.
The guards are... (1d8)
- Attentive - On guard, doing a good job
- Nervous - On guard, overly attentive and jumpy
- Distracted - Focused elsewhere (weird noise, disturbing thoughts, personal problems, daydreaming)
- Bored - Inattentive, lost in thought, maybe dozing
- Grumbling - "Why are we even out here?"
- Asleep - Physically there but unconscious
- Patrolling - Roll again and they're mobile
- Missing