Chaosmancer
Legend
I was more refering to the part of Thieves' Cant sign differing from guild to guild. In that case what rules apply to determining how likely a rogue is to understand the signs used by another guild or, in this case, a solitary woodsman in a tiny rural town and no connection to the outside world.
What the heck, I like a good debate.
Why does a woodsman with no interaction with the outside word know Thieves Cant... well to begin with, the same reason that said woodsman is delving a goblin cave with an Elvish Poet and the heir to a Human Noble house. Because he does have connections to the outside world, and therefore we can look to the outside world for some potential answers.
First thing that pops into my head? Smugglers. Oh sure, we think of smugglers as working in cities, but the goods have to get from city to city. Traveling along the roads may work for those who can forge official documents, but knowing the backways of the countryside is equally effective at getting from point A to point B without being noticed.
So perhaps your wilderness scout who spends their time in the deep parts of the countryside has encountered smugglers and bandits enough to recognize their signs. Those types of people mark their trails in ways consistent with Thieves Cant, so learning to read those signs might be enough proficiency to count as knowing the language in game terminology. (Because, the game is not grainy enough to recognize that knowing enough to be fluent in a language does not mean you know every word in the language)
In fact, the wilds of the typical DnD world are dangerous enough that perhaps there is a de facto alliance of scouts and thieves guilds and other such people who wouldn't want to hire a small army of caravan goods. Those people on the edges of society. People who live on the fringes tend to collect together for mutual protection, they learn the signs, they learn what to look for, Your typical Scout isn't the guy who goes to the local tavern every evening to hang with his buddies, he's a loner with a keen sense for when things don't match up. He's more likely to notice the gang signs that everyone else looks right past simply because a) he's less comfortable in the cradle of civilization and b) he's more likely to be paying attention so as to avoid trouble.
But, frankly, there could be a lot more explanations. This was just the first two that came to my mind.