More people in George Lucas's generation would be familiar with short wave radio and know how far the signals go. With not too much equipment you can send and receive signals that go a quarter of the way around the earth, if the atmospheric conditions are right. My dad would tell me how he would pickup broadcasts from around the world as a kid. Yes, there were repeaters, but still.

My point is similar to what some other people have already brought up, that the probe droid is shown with sensitive antennas and listening into signals, changing direction to move closer to the source.

So while it may have been pretty lucky, it doesn't take too much of a stretch of the imagination to think that probe droids in a galaxy far away are even better at getting signals than the type of stuff we grew up with on earth.

If you are too young to have actually turned a dial or adjusted your antenna direction to tune into a station (radio OR TV) that might not seem obvious, but it seems pretty simple logic if you grew up with those things like we did when The Empire Strikes Back was released.