When powering up or connecting the network cable all Emerson devices send a Gratuitous ARP Message. PAC Systems ETMs also send a message after a reset or hot insertion.
This message can be captured with a network analyzer like Wireshark to find the IP and MAC address of a unit.

Connect the PLC Ethernet Port directly to the PC (point to point). Run Wireshark and capture Ethernet traffic. First, try disconnecting and then reconnecting the network cable. In the Wireshark capture, watch out for a Gratuitous ARP message or an ARP Announcement. Sometimes, it may also be a normal ARP message. More general: Watch for a message where the source MAC address is not the PC.

If there is no such message, you may need to try a reset or hot insert the ETM, or power cycle the PLC.

The picture below shows a capture where the system was power cycled. Newer versions of Wireshark display this message as ARP Announcement.
Either case, the Device IP address is in the Source or Information field of the ARP message. The MAC address can be seen in the message details.