The key things you need to understand are:
- Whether the PSUs are internally isolated or grounded
- If isolated, whether there are any AC coupled transients from the mains side (many isolated SMPSs, have enough capacitive coupling that you can get 240V at ~10uA relative to ground)
- How/if the PLC connects to the network switch, and whether these connections are galvanically isolated.
It might be hard/impossible to definitively determine the answers to these questions, and the potential stray currents might play havoc with reliable system operation.
This is why cross-connecting the DC negatives to give a shared reference is such a common approach, but it may or may not be needed.
If you want to really see what is going on, take a two channel scope that can do "difference", then connect the tip of one probe to one negative and the other tip to the other negative (leave the shield clips unconnected). The signal you see is what you are minimizing by the cross-connect...