We retreated from a siege of a castle the other night and the retreating army had 600+ troops left, after the retreat they had 0. They kept a lot of equipment still, but lost 100% of the troops...
They should still keep some troops and some equipment, but obviously some people are going to desert, and some equipment may be left behind.
But if we're camping outside a castle in our siege camp, and we up and decide to leave...we're not surrendering to the enemy...we're simply walking away from where the siege camp was.