They'll have 100% a way of tracking post with a unique barcode especially as the majority of sorting is now automated. Be a bit like having a PNC check on all your post :(
Royal Mail don't look for drugs, it's not within their remit. But if LE find a half used pack of stamps in some packers house it's not going to be hard for them to find where the other half were sent.
everyone will most likely up their stealth, they still have noway of checking 99% of packages
who tf uses 2nd class stamps