'may contain' does mean whole pieces of nut. or whole ground nuts, esp with bread, cereal and chocolate, biscuits etc.
for instance the last i heard snicker bars were made on the same line as mars bars, and they have a may contain.
they switch from making a certain number of one chocolate bar, and clean the line. They clean the line with 're-work' chocolate which is pushed through the line, picking up bits of biscuits and nuts , and then, they put it BACK in the with the chocolate. Then they make the next batch of bars.( dairy milk bars, share with fruit and nut last i heard as well....)
In my opinon eating any form of chocolate with a nut warning on is a risk. just because one bar was safe last time, doesnt mean the next one is. so thats why we are very, very careful with food labeling. we read both the allergen box, and the ingreidiant list.
when they say nuts in the factory, does that mean the next line alongside the nut free ones, with staff working both at same time? or does that mean far away from the non nut stuff, or in seperate building? I think it can mean all of those, and its hard to judge from such a loose label.
sometimes we e-mail and ask a company about their products, and this is normally at christmas or easter, as am trying to find a new safe treat type of thing for him.