It is, if I recall correctly, an FS2 code bug where an "empty" turret still shows "something" and that "something" happens to just be "beam"
Not really an issue, I think there is some work being done on that, but it hasn't been committed yet because there is still some discussion about "Should it then be made Untargetable"? (but then what happens if it's a "stealth" turret that gets SEXP assigned a weapon?) or do we stuff an "Empty" into the display?
Good eye for catching that and reporting it though. It's glad to know it wasn't just shrugged off, but followed up on. Please, continue with any more that you find.