I got a Switched PDU off of eBay a few weeks ago and the seller didn’t know the IP but knew the login username and password ( apc/apc ) so I challenged myself to find the IP of the device. After a few minutes of thinking about it I ended up plugging in the PDU to a managed switch and mirroring the port to my notebook. Once ARP requests came out I was able to see what IP address the MAC address of the PDU its was asking for and set my local IP to something in the range.
After I changed my IP address I pinged the address and was able to login to the PDU. I have a screenshot of the traffic below.