Bandits are attacking caravans inside a hex. The only thing the party knows is the bandits aren't looking for treasure as they seem to be only attacking people in this one hex and aren't going full out to attack, caring little about whatever the caravan might be transporting.
The hex in question is just past a mountain range and next to a town, you they can semi-keep North/South/East/West without too much trouble. The land is a barren tundra, so it would be easy to get lost in the dunes and rolling hills of the landscape. The town has no info on where inside the hex the bandits are, as they are keeping all guards posted in case of an attack on the city, and thus haven't been exploring themselves.
What is the best way to have them try to find the bandits while exploring the hex and not just having them roll d% with a lowering DC as they keep exploring?
The hex is a mapping convenience not a physical feature. So the first thing I would do is work out the boundary of physical features and do a more detailed map of the interior.
Next think of some clues, such as the aftermath of attack sites, lookout points with evidence of recent activity etc. Consider sight lines, ambush sites and trail routes.
Consider what hints you could give characters with the appropriate skills to help focus the search or to rule out area of the map. E.g from this vantage point you can see a sheltered valley with a stream and some woodland that would be a good campsite. Looking East looks quite barren with little cover or game for the next 5 miles or so.
When you are done, go back and put in a lot more clues. What may be obvious to you may not be obvious to someone else. There are articles on the rule of 3 that may be worth reading (on phone so linking is too hard). Throw in a couple of animal encounters and some skill challenges to keep the interest up.
Perhaps you could create an outline map that the players fill in whilst exploring.
When playing, try and keep the pace up so it doesn't become boring.
Survival for tracking to trail the bandits to their lair.
Survival and/or knowledge geography could identify 'good' areas for a lair.
Knowledge Local could lead to someone who knows, perhaps a fence or ex member of gang.
A chance meeting while exploring blindly could lead to information (the old prospector that saw a group heading toward the black fang caverns)
I'd also be totally open to anything the players came up with on their own, giving a reasonable chance of success to their gambits.
To add to what I previously wrote. You should also consider what the bandits are doing. Do their lookouts spot the PCs standing on the top of nearby hills? Do they prepare an ambush? Having an ambush prepared might be worthwhile in any case. You spring the ambush if/when the players are getting bored or frustrated with the exploration as a way to move the game forward.
For the actual mechanics I would use something similar to the chase rules that various supplements (I think I've mostly used 3rd party such as Horns of the Hunted) have included. You can create certain features that the players encounter and play it out through skill checks or roleplaying. Straight barren tundra makes the game become "do we go around every hill or do we try and sneak to the tops of them to survey the landscape and potentially see the bandits first?" so adding more geographic features might be a good idea.
I would also recommend these resources for figuring out how to raise the stakes:
So if the players take too long, or are not stealthy/perceptive enough, the bandits can either ambush them or sneak around them entirely and conduct another raid on the caravans. So now the party feels much more pressure to pull the mission off. Just potentially rolling the Tension dice and them not knowing what might happen can really change the atmosphere of the session.
Thanks all for the help. Session went great. Players enjoyed it as far as one can enjoy a hunt. My main problem was Pathfinders stupidly low DCs. Day old tracks on snowy ground for a group of travelers is a DC 10. It was stupidly easy for the group to find the bandits, but then again the bandits went exactly hiding. I just stopped asking for rolls after a certain point because two of the (level 3) party had +7 to survival.
I also realize in the first post I was using gm knowledge. The bandits are within a hex, the players know they are within a rough area.