Use of RFID or suitable alternative to remotely communicate change in system state
You can get small FM or AM transmitters that can transmit a message when the output changes state. These can transmit to a central receiver and if the number of transmitters is small and the occurrences of the signals changing state is low you can probably "collect" this information but with the occasional data collision. A more sophisticated regime can be engineered where each transmitter transmits on a certain time frame and therefore collisions (if they happen) will not happen on the next time frame.

You would need a small MCU with each transmitter to encode an address and set up a transmit preamble and possibly append a CRC byte.

This sort of system works an I developed one for a bunch of food freezers in a warehouse. If the freezer got a little too warm it would transmit that fact to a central receiver and this would alert staff to take some corrective action.

Uing RFID is not going to work because the distance the tag has to be from the "reader" is quite small i.e. a few centimetres.