I've somewhat recently been noticing that Hummingbird's been placing the define('WP_CACHE', true); line too far down in the wp-config file for Hummingbird to recognize it.
Most of the time I can clear the cache and the warning will go away, but it won't always. For example, I tried clicking on the "clear cache" button on the Page Caching page a few times and nothing. I clicked the larger "clear cache" dropdown/insert above and it worked this time.
I'm not sure if it's coincidental or if there's an actual effect, but I've found that it tends to work when I clear the cache after I've removed all of the auto insert info from the wp-config area.