They just pick it up mate, if you encourage or praise them too much it can lead to false marking.
how do you tell,does their body language change,my lurcher is only 5 1/2 mths old but my cocker ,when we are out,some burrows she will just ignore other she will stick her head or body down the hole as far as she can have a good then come back up,look at me then back for a sniff again,is this her marking?cheers......john
Sounds like it could be - test her with the ferrets, you'll soon know.