This phenomenon has happened to me many times and on more than one computer:
I install Battle.net, I install WoW, I open WoW with the Battle.net app for months.
Then, one day, like today, the Battle.net app tells me I need to install WoW. I can still open WoW from where it is installed, and I can still play the game fine. When I try the “Locate the game.” function near the install button and I go to the folder that is containing the program, “Locate the game.” cannot see the executable file.