Well firstly, i am terribly sorry to hear this. Multiple deaths always concern me, sadly a loss happens but if more than one does short of predators something has to be happening, now here lies in the trouble, what? From what i am gathering from you they are symptomless, do you have Storey's guide to ducks? he lists ailments in it, perhaps worth a skim.
I personally, have no suggestions, have you brought any new birds in as of late? being you were just inspected i sincerely doubt it's your care. My ducks are always in the mud and we keep other livestock which they are around too.
Sadly, your best bet is with this testing being done, i hope it provides you with some answers.