That's an interesting one Tom.
Surely there would be some sort of a narrow-band notch filter that could be fitted to the Opener Reciever units, to prevent this sort of thing happening.
Oddly enough, we had the same problem here with our Fire station Pager transmitter when it was first installed, it used to open all the garage doors on the block.
We had a local technician make up a heap of filters for the units and that seemed to cure things.
What a lot of people don't understand, is the fact that it isn't the transmitting equipment that is faulty, it's the in-ability of the reciever circuitry to filter out the unwanted signal.
This happens a lot when appliances are built to a (usually as low as possible) price.
Let's face it, these days if you're not young, you're old - Red Green