This was just a siren then? Makes sense.

Smart system, really. Very simple to install/maintain and very simple to have a robust transmitter infrastructure around.

If you just substitute your 4-digit code for a longer, say, 64, digit one with some time-based component, and perhaps shift the required frequency too based on time, you actually end up with a very robust very simple system.

I disagree. The frequency hopping would make it harder for a signal jammer to prevent a legitimate transmission. If you can't predict the frequency on which the transmission is being listened for, you'd need to jam a broader spectrum, needing much more power to perform the attack.