This appears to be a problem with this gen Odyssey. My 2007 has rusted through in this spot on both sides. Our van is always garaged, and has 225K miles on it.
Everything else on the exterior looks good, no rust, and paint still good. Wondering if Honda has TSB or other out on this issue. Definetely rusted through from the inside out. I've seen other cars do this at this seem as well. water in behind and in between, rusting it out from the inside. Bad Design. I took one side apart, ground it all down as best i could inside and out, bondo-ed and repainted and rust came back through in a few weeks. Seen this same problem on several different posts now.