A leader is besieging with only mercenaries. They then depart due to lack of pay. The besieged are therefore faced by only the leader and so the siege is broken. Is the leader captured or does he go to the nearest friendly army - or place? Or does he just stay outside the key/fortress?
The rules mention leaders being captured only after losing in a field battle or assault. There has been no field battle or assault.
Treat this case as if the leader had been leading an assault and his entire force had been eliminated. So:
"If all attacking land units are eliminated, surviving attacking army leaders are placed in the nearest fortified space or their capital as described in Step 6 of the Peace Segment Procedure (Section 9.3)."