The ip-address of a pcap file “question”

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Open the capture file and then use the menu item Statistics -> Capture File Properties. Amongst the data displayed is info about the capture. Unfortunately it doesn't contain any info about the "user" who made the capture.

If you display the Statistics -> Conversations dialog you may be able to identify the device sending the most packets which is likely what you want.

Wireshark does not need an IP address to capture traffic as it is just listening to the selected NIC interface. The reason why I am saying this is when you use a port mirror on a switch the interface will be active but you cannot seize an IP address from the DHCP server, as a port mirror is just one way traffic to the Wirehark PC. The same applies when you are using a TAP which allows for inline capturing of traffic