Tampered goods!

I bought a box of Nestlē cereal bars which supposed to contain SIX bars, only to get home and find that someone (not sure whether it's staff or customers) had already helped themselves to one and nicely closed the box to give the impression that it was sealed! I always check my products before buying it but the way this box was neatly folded back into place made it impossible for me to have foreseen this theft! I am truly disgusted as I can't even return the product because how will I prove that the box was open and missing a cereal bar when I bought it?? Pick n pay this is unacceptable!!