Officially a 240z owner!

Best put up some pics of what you have found. If it's like mine, http://www.classiczcars.com/photopost/showphoto.php?photo=25915&ppuser=1608&sl=2 you'll need to have a real close look about. All I saw was a small dark match of underseal and when I poked it with a screwdriver, it shot through. The other side of this plate was rusted out as was the inner section of the frame rail. Remember that this part is two layers thick with no rust proofing. As it is in a ever moist area, if the seal isn't perfect, water will track through. I was lucky that the frame rail wasn't too bad, I had get some it replaced.

Take off the front sway bar mounts and have a look in there too.

Also have real good poke about under the battery tray from the wheel well with attention also to the seam where it meets the firewall/footwell area.

Also check the front radiator support.

Have a look at the pics in my gallery of what rust can be found in a seemingly "minimal rust" car. I'd suggest to anyone to drill some 19mm holes in each footwell so you can access the area with occasional rust preventatives. Checkj out the first picture in my gallery.