'force field' type alarm?

[ja421]

years ago; I had a circuit that would sense when anything (human, animal, or metal/car) got near it... Of course, I can't find it now.
I believe it worked off a low power radio receiver to sense changes as things approached it.  Sort of like a proximity sensor in 360 degrees.
I've searched in vane to find a similar circuit (pre made or not). 
The idea is to give a beep if a car gets too close to a motorcycle while parked (to avoid it from being hit/knocked over, which has happened from people 'not seeing it' or thinking their bumper could clear it)
Since I'm after detecting cars, not people, I don't think a bunch of PIR sensors are a solution...

Any ideas?
Thanks.

[NottheDan]

An RF proximity sensor? Google gives quite a few results to those keywords. Or do you mean something else?

[Dataforensics]

Perhaps get an ultrasonic parking sensor kit as intended for cars. One of the simple ones with just an led bargraph, then pick off the signal for the warning distance required.

[ja421]

An RF proximity sensor? Google gives quite a few results to those keywords. Or do you mean something else?

It is amazing how you can search for something and not find it because you don't know the proper term to search for... Like asking a small child to look up "Knee" -- if they don't know it starts with K, it would take them forever to go thru the dictionary page by page. (of course, that dates me)
Thanks -- This looks like what I am after.

Perhaps get an ultrasonic parking sensor kit as intended for cars. One of the simple ones with just an led bargraph, then pick off the signal for the warning distance required.

I thought of that; but they are fairly directional.

I think "RF proximity sensor" is what I'm after....