This likely means that a user visiting did not leave a USER_AGENT header, most likely meaning it was not a real person visiting your page. If the USER_AGENT header wasn't left, it probably means that they weren't using an actual browser to visit your site, more a piece of code.
Pretty amateur of whoever made it not to leave that header when requesting the page though.

As for your captcha problems, changing the captcha on your faucet every once and a while doesn't hurt. Maybe changing it between Funcaptcha and AreYouAHuman, as they are two somewhat bot-proof captchas.