Back in the 90's I worked for a company that built coin changers and acceptors. Even back then they were using a more advanced method than the one you describe. The acceptor was actually the part that determined what type of coin was inserted and would reject it if it was a slug. The acceptor worked on the same principle as a metal detector. Since coins are made up of a unique mix of metals the acceptors could be "tuned" to accept a certain coin by dropping versions of that kind from different years and differing conditions. This would create a range of values for each coin. When a coin is dropped into the acceptor it gets a reading of the coin. If it is in the range of one of the coins the acceptor is programmed to accept then the coin is routed to the proper coin tube or the cash box if the tube is full. Otherwise it will be rejected as a slug. The acceptor is actually plugged into the top of the changer as a complete unit. For what it's worth I did see an acceptor there one day that worked similar to the way you described, but I believe it was from the 70's or early 80's.

Catcher in the what!?

How about "Those who forget history are doomed to repeat it".

I agree.

Of course there is also the possibility that they never developed radio communications to begin with. They could have developed a completely different technology for long range communications.

I do believe it is likely there is life elsewhere in the universe, but if they never developed radio, or stopped using it eons ago, we will never detect them this way.