Your best bet for success is sniffing data on an RS-485 network. For everything else, who knows.
TL;DR
It's not a big deal to eavesdrop on RS-485; RS-232 is a bit more complicated, but doable; ethernet/TCP/IP would require a sniffer (ethernet adapter in promiscuous mode), again fairly straightforward for RaspberryPI or PC (Linux certainly, probably Windows too), but I doubt it could be done on a PLC.
The headache would be the parser: because Modbus protocol is request-reply, using any existing Modbus RTU master PLC instruction would be problematic. Modbus TCP is possible: TCP servers can in principle, and often do, handle multiple clients, so it would depend on the Modbus TCP implementation.
That said, if you are reduced to sniffing traffic, the Modbus protocol is fairly simple to decode. The main requirement would be knowing up front which Modbus server (IP address and/or node address), and which messages, you were looking for.
And while that's all possible, for the amount of effort it would take it may be preferable to instead reconfigure the entire network to have one client that buffers all the data in one place, and that also runs a server that every other client queries. This is pie in the sky, and even this may not be possible with your existing third-party setup; it would also introduce delays that may not be acceptable.
Sniffing data on an existing Modbus RTU server on RS-485 would probably need a secont serial port, because the existing port is already being used by the server instruction.