@discographer-- It was probably a case of cross contamination. If the chocolates are produced in a facility with non-halal items, contamination can occur. That's why some Muslim organizations test popular food items for non-halal ingredients. I suggest checking their findings to determine which chocolates are halal.
I actually think that mass produces chocolates are safer than chocolates made by small confectioneries. Confectioneries tend to experiment a lot with their ingredients and a lot of non-halal ingredients can find their way. My coworker's husband is a chocolatier and he makes some strange chocolates sometimes. He even had one with worm and alcohol in it, to mimic a tequila drink that is served with a worm on the bottom of the glass apparently. I think that would definitely be a nightmare for a Muslim!