Update: I figured it out -- it's from another Left-click during guidance (the shoot button.) This can happen by simply double-clicking during the ADS (scope-in) animation to fire as soon as it finishes, and it buffers the second input and puts it through after the rocket fires. Please fix this so that the "shoot" button does NOT turn into a "reload" button during operation.
Double-presses during "buffer" periods before an action is available is very commonplace in games to make sure an input-window is reached (in-case the first press/click was too early) and should be accounted for as a normal input feature.