All RS485 drivers I know are specified to allow the output to swing above and below the rails, which means, that connecting an unpowered RS485 transceiver won't do any harm and allows the bus to operate regularly. The classical SN75176 e.g. is specfied with -10/+15V at the bus terminals.
A special problem arises, if only the terminating nodes implement bias resistors to set the idle state, as it is e.g. the case with PROFIBUS networks. If the node(s) providing the bias voltage are unsupplied, the bus may fail, because some peers see a break state.
P.S.: CAN drivers have basically a similar voltage range specification, but the bus may be more sensitive to irregular behaving devices due to the recessive "1" state.