I don't have the same version but I'd check the events for overwrite events or check if you don't allow incoming rfc 1918 you may have some traffic coming from the vpn and being translated in as a rfc 1918 addresses.

The only thing I can think would be searching on the events log file and doing some tests after hours or off peak

Try emulate the situation while the traffic is low and would be a lot easier to read the logs.