hmm, tis a new one on me too....
It basically means you are on an encrypted page (with HTTPS at the start rather than the usual HTTP) but something on the page like an image is being called from another server.
I've given the pages a thorough checking over and can't see anything strange or replicate the problem
All the payment details are taken on independant secure servers (paypal and SECpay respectively) so they are always completely covered and secure irrespective of what's going on on our website.
Anyway, I'll keep an eye out, but do let me know if you get any further funnies
cheers,