Kathy, just some thoughts as I read your message.  Perhaps God was answering you by showing you how deceitful Chris is being in order to pass the drug tests.  Perhaps this knowledge will help you somewhere down the line, when this have to either throw him out when he turns eighteen, or deal with an arrest at some point.  So although the surprise test didn't result in his being held accountable (yet) for his actions, it did give you guys the chance to learn why he passed, and it wasn't because he wasn't using.

As for the monitoring, I'd continue for now.  You guys are protecting yourselves and your family by informing yourselves about what he is and isn't doing.  I wonder if you should let the probation officer know what you heard about the surprise drug test and why he passed.  Maybe someone needs to go into the bathroom with him and watch to make sure the test is done correctly.  Actually, I always thought that's what they did.  Good luck, and keep us posted.  My heart aches for you guys and what you're going through.

Hugs and prayers,

Peggy