Continuing the theory, if you had one of these RS485 modules (there are others as well):
If you connected the RE & DE pins to 0v, then the module would be in receive mode. The RO pin will output any data that's on the RS485 bus. When the 485 bus is idle, I think the RO pin will be high. At the first sign of any 485 activity, then the RO pin will go low (i.e the start bit of the first byte of data).
You could use this high to low transition to trigger a monostable timer which in turn drives a relay. You could also use something like this module that I found on AliExpress:
The description seems to indicate that it will trigger the built-in timer when the trig pin goes low, the timer is triggered and the relay energises for a pre-determined time. The original text is:
Providing a low pulse signal, or touch trigger key, relay and indicator lights, the output of the state: normally open (NO) and Common (COM) connected, normally closed (NC) and the common terminal (COM) is disconnected. After 0-24 seconds (this time can be adjusted according to your needs) relay off, back to the state before the trigger.