If they're in Syria, we know it. In fact, a convoy of chemical agents was on the way to Amman from Damascus a couple of months ago, and it was intercepted by Jordanian security.
I remember reading an article shortly after the war started that stated our spooks observed a convoy of some 1400 vehicles streaming towards Syria. A lot of these vehicles contained people fleeing the impending war, but some were 18 wheelers and other trucks carrying who knows what.
Another article I read some time back stated that France and Russia both sold tunnel boring equipment to Iraq in the 1980's. It was supposed to be used to build a subway in Baghdad. To my knowledge those machines have never been found.
In a related note, back before the 1st Gulf War a British MP went to visit Saddam in an attempt to convince him to pull back from Kuwait. This guy said he entered an elevator in one of Saddams palaces and decended so deep that his ears popped twice before the elevator stopped.
Some of the stuff Iraq had undoubtedly got out, probably to Syria and Lebanon, giving us a good excuse to "cleanse" the Bakka Valley when the time is right. As far as the rest, find the tunnel boring machines and the tunnels and you will likely find the WMD's too.