If you see the warning “This is a non-secure form. This form will be sent in a way that is not secure. Are you sure you want to send it?” after a PayPal payment is made and the user is being redirected back to your site; this is caused by your site not using an SSL certificate – (using HTTP instead of HTTPS).
There isn’t actually any sensitive data being sent at this point so I don’t really know why PayPal does this, however there is no way to tun it off. The only solution is to use have an SSL Certificate on your site so that it is HTTPS.