This sort of mechanic is already covered with KEEP WATCH/LOOKOUT designation. Details on this are on page 134-135 of the Alpha PDF. Succeeding on a KEEP WATCH allows the group to see any threats from a safe distance and have a chance to avoid them. How that is resolved would be left up to the players and GM. As a GM you could say that the PCs must succeed on a group STEALTH roll against the threats SCOUTING to continue to move through the current hex. Alternatively, the PCs could opt to back track and move through surrounding hexes but that will cost them time. I'm sure you can come up with other ways to resolve this as well.