Since you're grasping at straws here, I've got a bit of an off the wall suggestion. When sweeping for eavesdropping devices we use a special probe to search for audio that is passing along pipe etc. It got me to thinking that maybe you could create that situation to your advantage. I took a transducer and placed a low frequency bass hum into a ground wire, then had one of the guys here take a shotgun mic and see if he could find it using it like a stethescope. He could actually hear it pretty well.

Keep in mind that the structure here was wood however. If it was a steel structure I imagine the hum would radiate everywhere.

I know it may be a long shot. It's just the kind of crazy thing I'd probably try if someone told me I couldn't do it. :idea: