‘Matters of Testing’ gives this example of sub-frame corrosion.
As with many modern vehicles, this sub-frame was obscured from view by plastic shrouds and the under-tray. It was not until these had been removed that the full extent of the problem could be seen. Both sides of the sub-frame had completely corroded away. The corrosion itself was not visible due to the covers but the sub-frame had been seen to move leading to the problem being detected.
The problem was found by Tim Jones on a Vauxhall Zafira. Tim said he was using the shaker plates on his ATL and initially thought the observed play was because of wear in the wishbone bushes until closer inspection showed it to actually be the sub-frame moving.