are the sizes of the chocolate pieces consistent? or you run multi-sized products-if these are consistent, here is one way I had done it in a similar product and didn't meet any resistance from auditors
pre-make the chocolate by embed the test pieces inside- colour code the pieces(mistake-proof) and label them . this ensures if real pieces are in the middle then you can catch them .
make new samples every month and keep the metal test piece calibration records etc handy. If there is chances the choc will melt then this possibly wont work
To answer your validation question: validation you generally don't do on a daily basis and here is steps to do one
for each test run the test pieces 20 times and they should reject all 20 times.
Pick your worst case scenario- eg SS is difficult to pick up, metal detector # 3 had more issues this year etc , used your smallest box compared to aperture
1) place test pieces on the top of the product
2) place test pieces on the middle of the product
3) place on the bottom of the product
that's your vertical validation
repeat this on the horizontal side
then do a leading, trailing check
check your buzzer, your andon lights, lock on your rejection box, cleanliness of the belt, vibration and voila- you are done
if you get a single failure then - this is a fail- adjust and repeat.