I happened to be in a position to observe a friend's pump yesterday and noticed something "strange." (emphasis will be explained later)
I knew that the pump was running because I could see clear evidence of the fact, but the pump strainer basket appeared to be empty. I came this close to making an obviously stupid statement to my friend to alert him to the fact that his pump was running dry, when my logic kicked in. The pump couldn't be dry because I could clearly see the output. So upon closer inspection I was able to detect water running through it only by the very slight quiver of a leaf caught in the basket.
I was thrown because I have become so accustomed to my own system that clearly has a suction side leak. I had never seen a properly functioning system before! (That's the punch line to this poorly constructed story, BTW.)
Hopefully it will be resolved by a little lube of the lid o-ring. But I just thought it was interesting to see how a pump/system is supposed to look when operating...