I won't call it. Simply because it really doesn't matter. It doesn't give the guy an unfair advantage at all. And more imortantly it will create a real unpleasant atmosphere in the group.
A few years back in my club championship I ended up winning by 1 shot. However after the round, the guy I beat (not my marker, the 3rd guy in he group) said I should be penalized for picking up my ball from the hazard with my club way back on hole 4. I was obviously not going to play the ball, it was 2 feet under the surface, I just didn't want to lose an expensive ball when I easily could pick it out with my wedge. What an a**!. And to mention it after the round was over, when it was clear that I had won.... He said the reason wasn't that he was 1 stroke back, but rather so that "I could learn".
In the end I wasn't penalized and the guy looked like a bitter fool