NEED HELP - Is this effects loop broken or just the way it works?

[DeyBwah]

"When I plug in my Pod Pro FX processor to the effects loop, I turn all levels on the amp head and the Pod Pro to 0. I should have no signal when my volume on my guitar is on. This is not the case, when I barely turn the volume on the guitar up, it is getting a very loud signal while all the levels on the Pod Pro and the amp head are turned down to 0. There should be no signal at all when I do this right? I have a Marshall head I play through and every thing works fine when I play through it , so i assure you it is not my guitar or the Pod Pro."

I sold a Warhead head to this guy and that's his latest correspondence.

I've never used an FX loop before so I don't know if it's a malfunction or the guy doesn't know what he's doing. In any case, can someone who knows how FX loops work chime in with their thoughts and expertise?