Unless there is a source of water constantly coming into that line, a void (air) would be created in the line from the loss of water.
Not true UNLESS air can be sucked into the line. And if that happens, there is a water leak in most cases and with the check valve at the tank, you can't tell there is a leak EXCEPT if there is air in the lines.
Because of a check valve at the tank, no water is coming back into the line from the tank, so an air pocket can be created. The water column will drop from the top down. So it really doesn't matter if the leak is underwater or not.
An air pocket can not be created UNLESS there is an air leak AND that air leak HAS TO BE ABOVE WATER or water would be sucked into the line instead of AIR.
I can see this not happening when the column of water in the well is equal or greater than the pressure of the water in the line. In that case water would not leave the line.
With a check valve in/on the pump, or a foot valve, and a check valve on the tank tee, the pressure in the line is HIGHER than the water in the well.
So pressure has to leaked off to 0 AND THEN there has to be a water leak BELOW that leak, before any air can be sucked into the line AND the leak has to be above water or water is sucked into the line instead of air. That's TWO LEAKS that will not be seen with a check valve on the tank tee.
What i am trying to explain is that you can in fact have an indication that there is a leak in the line when a check valve is at the tank.
Not if the leak that allows air to be sucked into the line IS BELOW THE WATER LEVEL IN THE WELL! And a second leak below that one.
I could care less where someone installs a check valve but it needs to be noted that you can have an indication of a leak with or without a check valve at your tank and one can be more obvious to the other.
It sounds to me like you like selling your customers extra check valves. As to the rest, SEE ABOVE, including the posts by other people here telling you the same thing.