thank you for your reply.
I can't think of any reason why the Fortinet would poll the APC using SNMP. Essentially both devices support SNMP as an "agent/monitored device". They can have different SNMP communities, that's not a problem and the Fortigate shouldn't be polling the device (I even doubt it has the capabilities to do so).
Could it be that you're seeing this connection as coming from the Fortinet because of an inbound NAT which masquerades the real IP of the device trying to poll the APC? You may want to set-up a "Filter" or "Live View" (if the Fortigate has something like this) to view all port 161 traffic on the firewall (to see the source interface if it's coming from somewhere else)
Lastly, you could also pause the "Root" element of your PRTG for a couple of minutes to check if the messages continue or stop. This will tell you if PRTG is involved in any way.
Luciano Lingnau [Paessler Support]