Is sniffing possible on a wired network, assuming it is physically secure?

Is sniffing possible on a wired network, assuming it is physically secure?

I had a security concern with an unknown device on the network.

If Wi-Fi is disabled and the network is strictly ethernet, assuming that no malware is deployed and it cannot be physically accessed by an attacker, can packets still be sniffed somehow?

This may be overkill, but I want to learn more about network security and ensure that we're safe.

Any insight is appreciated!