As luck would have it, the scenario I described actually came up. I'm a vassal of Vlandia, at war with Battania, and was hanging out in Western Vlandia trying to recruit some decent troops. Someone had an "Escort Merchant Caravan" mission, and I figured why not, we both pretty much have to be going in the same direction, right?
Turned out this frickin' caravan was going to the ultimate destination of that Sturgian town in the far northeast, so it was an escort mission across the whole damn map. (The payout was quite generous, though, I have to admit.)
Anyhow, as I was diligently chasing away bandits and looters who might threaten the caravan, Caladog pops up right in my path, hanging out with just 14 guys near one of the south-central Imperial cities. I snapped him up easily enough, but I wasn't anywhere near a Vlandian castle or town, and had to stick close to the caravan I was escorting. I had to keep him with my army. But I did, for a much longer time than expected! I took the "northern route" home, through Sturgia, and he didn't escape until we were in Western Sturgia. Not sure how many days it was, but essentially it took the length of a half-circumnavigation of the map before he got away.