I doubt they are on the lookout for mines. As far as they know the Mill just burned down.
How would they even know that? but nether less disturbing the ground seems like a good idea for the minutes afterwards when they realize what's happend or if they're on high alert for another reason. Every little helps
There are two patterns we could use while disturbing the earth: one covering a large section of road before and after the mines, so that there is no sudden change to give them any warning; and one covering the areas after and beside the mines, so after hitting the first mine they believe that there are many, many more. The first would be good or useless depending on how perceptive they are; some of those guards experienced in war might immediately stop the caravan at the disturbed earth, call for ambush alert, and pull out a long poll to set mines off prematurely. The average guard would be too busy staring down the trees and bushes. If we do the second, we will probably want to place mines where the caravaners will be most likely to try calling the bluff (or grit their teeth and decide to go through the minefield anyway), and avoid situations leading to the thoughts "if they have so many mines, why didn't they put any
here" and "why would they place mines so close to themselves" (which comes down to evenly distrubing the "mines" over the C shape and the party being obviously very careful were they step). However, when we blitz them from behind while in this C of "mines" and they feel cornered, it could mean a nastier fight just as easily as surrender. It would depend on whether we want to intimidate them or kill them all before they can react. We should probably put significant effort into making a small path through the "minefield" that looks like a route those laying the mines used and forgot to cover up, but with one mine along it for the poor one who decides to flee.