Exactly that, yes! IR receivers outside every exterior door to the building, and IR receivers in the elevators to control access on a floor-by-floor basis.
The fobs were visible by an IR camera (including the average smartphone) and could trivially be decoded as a short bit sequence with an IR sensor wired into a microphone jack, as the bit pattern was transmitted at ~audio rates.
The fobs were visible by an IR camera (including the average smartphone) and could trivially be decoded as a short bit sequence with an IR sensor wired into a microphone jack, as the bit pattern was transmitted at ~audio rates.