[Plugin: Really Simple CAPTCHA] Captcha files keep disappearing

I am using Really Simple Captcha 1.0 with Contact Form 7 (1.9.5.1) on WordPress 2.7.1.

I’m using the standard directory to hold the Captcha files i.e. /wp-content/uploads/wpcf7_captcha/ and when I create a new Captcha tag for ContactForm, I see new pairs of files appear in this folder OK.

But every so often, all the files disappear from this folder, which of course breaks the Captchas in my forms, so I have to create new Captcha tags for my forms.

I don’t understand what might be deleting these files, so can anybody suggest why this might be happening or where I should look to find out what’s going on?

Thanks!