Maybe the Colonel has a broken leg or is too fat to march. Maybe he rides in his jeep. Maybe two soldiers carry him in a hammock. Perhaps he is staying behind with a girlfriend. Maybe, like Simon Bolivar he had to jump out of a window and ride away on a horse before the husband found him.
Well, there is a better verb to use for "leave" than marchar. In this case, perhaps the colonel is simply watching the troops as they march
Would walk also be acceptable??? I guess caminar would be better buuuutt....