Belt stretch or a faulty tensioner?

the only thing I've ever seen wear out on the tensioner is the pully bearings, although I have seen a spring crack, but you would definately notice that

Zombietime my belt does the same thing. I put on an 85" belt and it will be right inbetween the arrows. After about two days it is right back out of range. Mine has been like that a couple of years now with no problems. If I try to go shorter to an 84.5" belt my engine surges on start up really bad like the belt is too tight. I would just keep a watch on the belt a few days and if your engine is satying cool and your alt. is charging and you hear no belt squeal I wouldn't worry.