With IPHost Network Monitor you can run simple snmp requests against a Fortinet device in your network.
fnUserName
fn User Name
1.3.6.1.4.1.12356.1.102.2.1.2
A name of user account
Back to FORTINET-MIB-280 MIB page.
IPHost Network monitor allows you to monitor fnUserName on Fortinet device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Fortinet switches right now.