Brigands

When you disband an army, there is a less than 50 percent chance that up to 25 percent of the disbanded troops will form themselves into a band of brigands. This brigand army (or "company" as they were known) will be led by an NPC and will randomly move around and pillage fiefs until they disband from combat losses and attrition (just like any other army.) You can go after a brigand company just like you would an enemy army. You must use the "Brigand Combat" option on the "Dirty Deeds" menu for this. You will also find there an option to buy the brigands off (in effect, hire the NPC brigand leader as an NPC and the brigand troops just like any other troops recruited.) The other option is to do battle with the brigands. This can be tricky, as the brigands will always retreat if attacked. But if your force is much larger and/or your army leadership is very good, you'll be able to trap and eliminate the brigands.

Historically, and in the game, brigands became a major problem in the game. The English purposely disbanded their armies (usually in the Fall, as it was too risky to keep fighting in the Winter) deep in French territory. While most of the troops would make their way back to England, many would form bands of brigands and continue pillaging France on their own. Since each successful pillage does some damage to the fief, this brigand activity did additional damage to the French without costing the English anything.