OK I feel a little bit silly now. I checked using the LiveHeaders stuff in Mozilla, very useful, thank you.

Why do I feel silly? Despite the fact that the page content says "404", the header says "202". So I do in fact always get a 202, but in a regular browser I see a big fat 404 message, which tricked my feeble little brain into thinking that the server was returning the 404 response code.

The mystery of why it should be returned at all is still unsolved. I'll ask the UNIX boys upstairs.