On my 207 282, the shelf immediately below the window is dry and so is that same shelf shown in your picture. I'm more predisposed to think it is a window leak. If you've led a good life, maybe just the window shelves are rotted. There are some stainless brackets that support the decorative box that surrounds the window. If those brackets and or bolts are rusted, that might be further evidence of a window leak.
Rob