Saimon Eri: I'll bet he ran away to have his own little adventures because he thought army life was hard, boring, and dangerous.
Saimon Jirou: I won't beleaguer you with the details, but I was merely baiting those crooks in when I was captured the previous two times.
How to complete the Farmer's Treasure world quest Saimon Jirou asks you to look for old stone slates.