I have also recently had this message pop up on my dash. My truck has just under 14,000 miles on it. I thought it was odd that it was telling me this with so few miles on it. It made me wonder if there was some sort of "reminder" pre programmed into the computer to remind you at a certain mileage/hour or if their is some sort of sensor being used to trigger the warning.
Idk, I am just skeptical about stuff like this because dealerships/Ford make lots of money via service so I can see this being some sort of bs to get consumers to come in and get it checked out. Perfect example with the experience that the OP had. Me being the skeptic I am is not so sure that it was a case of "unknowingly" using the wrong test as much as it could have been a case of "welp, now we are going to need to sell you this additive, and a half hour of labor, flush the whole coolant system, etc. etc.. Most people would not have even asked to see the results of the test, and if they did, would not question whether or not it was the right test.
I will most likely end up testing myself first, then taking it to the dealer to see what results they come up with. I want to take it seriously since coolant stability is especially important in diesels but I also want to make sure my current dealership service department is more reliable/honest than the last one I dealt with.

 

There is no sensor triggering the alert. It is simply built into the computer to have you check at certain intervals. Most tests come back good.

 

Air is a villain in our coolant systems. I half believe that the message to check coolant, is to just to make sure your coolant level is high enough so that air doesn't get into coolant system.

Plus the test strips will warn you if somebody put the wrong kind of coolant in your system.

But like the Seabee above said, It's in the computer to check at certain mileage intervals. Not because a sensor found a fault

 

This "Check Coolant" is a diesel thing and not a gasser check. Here is what the 2015 Diesel Supplement says. I believe this was similar to checking the old 7.3's for the CCA's to prevent rusting and cavitation.

Checking Coolant Corrosion Inhibitor Additive Strength

At specific mileage intervals of 15000 miles (24000 km), as listed in the
scheduled maintenance information chapter, the coolant corrosion inhibitor
additive should be checked. The optional information display, if equipped, will also display the message CHECK COOLANT ADDITIVE at this time. The purpose of checking is to verify the correct engine coolant concentration (freeze point protection) and corrosion inhibitor additive level (strength) of the coolant for maximum engine performance and protection. Three products are available to confirm the life and health of the coolant: one tool, a test kit and a coolant inhibitor additive: