Arduino RS485 Modbus ' Listen mode' HELP !!

Hi all,  I am new to  Github , but not to coding and networks.   

However I am struggling for a solution to what appears in the outset, to be a very simple task. 

I have a Solis hybrid Solar PV inverter which has a remotely mounted current transfomer (CT) transmission unit that speaks modbus over RS485 back  to the inverter.   The inverter ( master ) is sending a request for 30   x   16 bit registers  from  address 31000 to 31029.  The CT unit ( slave) replies with the data which includes 31013 which the the only piece of the data ( power exported ) I need.  

So my question is :-  Can the arduino be configured to’ listen’ for the response from the slave, identify register 31013 and make that data availble ??

I cannot make my arduino a master , as only one master is allowed per RS485 network.  Similarly, I can’t make it a slave, as it needs to gather data rather than send it.   So … is there a listen  / monitor mode. ??

Any thoughts or example code are very welcome.