serial connection hackable
And this is the reason why there used to be a backdoor by using the direct serial connection.
I the "trunk" line feeding the switch is on a single VLAN and not showing up in a traceroute, it can be thought to be a dumb switch. Most likely it has a 802.1q trunk line and may require additional network tracing. This tracing may include visiting all devices connecting to the switch and seeing the settings there.
Ah, the good old days of tracing a ethernet cable through the network floor......