If the normal testing process is to occasionally take positive samples home for the weekend and store in a refrigerator, which would also require no one else having access to the contents in the refrigerator--then I guess the lab sample custodian did nothing wrong. If that is not the normal procedure or even an approved alternative, then taking the sample home for the weekend (instead of sending it out via Fed-Ex, or storing the sample on premises in an approved limited-access storage container), in effect, breaks the chain of custody.