You have most probably enabled the HTTP Proxy on one of your external interfaces. By external interfaces I mean interfaces that have a routable IP assigned to them (as primary IP or as an alias).

You must configure the HTTP Proxy only for your internal/private networks. When activating the proxy for an external interface, you practically invite others to use your UTM as an open proxy. After a couple of days you will see a lot of random IP addresses navigating the internet using your IP address and your bandwidth.

Solution: Disable the HTTP Proxy on the public interface(s).