Last night I was reading a webpage (sorry, I closed the window and forgot where I found it) where a senior Chinese veterinarian (reporting under an alias) reported that authorities have been hiding a widespread poultry epidemic since last year. The area of the outbreak coincides with the areas where the inital human H7N9 cases appeared. In Shaanxi province IIRC, he mentions that the normal 20 million poutry population is down to 7 or 8 million (however, it's not clear if he means that they all died from the bird flu or if this figure includes flocks culled by the authorities and never reported in the media).

It sounds plausible to me. The mass cullings that have been going on periodically for more than a decade over bird flu fears have taken a heavy toll on poultry farmers and local economies. Locals want as little of it as possible since everyone loses out. Political advancement by local Chinese authorities often hinge on promoting good news and minimizing or hiding bad news in your area (well, this happens everywhere but it a particular problem in China). China is so spread out that Beijing often has little control over what happens on a day-to-day basis out in the provinces. A lot of what Beijing gets blamed for is decided at the local/regional level.

Anyway, so it's possible that the H7N9 virus has been around longer than the apparently sudden emergence in humans last month. It does raise interesting questions about whether people have been sick of it earlier, but was being mistaken for something else.

This is the only whiff of this story that I've seen anywhere, so I can't corroborate with any other sources.