Balk???
 

Runners on 1st and 2nd. Left handed pitcher comes set. Without moving the location of his hands at all, the pitcher merely flexes his glove (open and closes it). Hands do not move but defintiely there was movement to his glove. No change in location of his hands, no start of pitch, and no start of a move to 1st base. Just his glove flexed!! Balk or no balk???

Well, it is kind of hard to believe that a glove moves on its own. So it pretty much means that there was some hand movement. Now to what extent hand movement is allowed is truly a "had to be there situation".

I have seen many pichers after having come set have some movement of the hands but, I never have called or seen a balk called because of it.

The pitcher is allowed to adjust the ball within the glove. I would regard the flexing of the glove as part of that adjustment. Legal.

You could also consult the COMMENT in 8.05 that says that if the umpire has any doubt as to whether a particular movement is legal, he should consider whether it is intended to deceive the runner(s). In this case, it's hard to see how this could deceive anyone.

To pick a booger or not to pick a booger.

I'd take the opposite approach, here (with the caveat that it is HTBT). Why would the pitcher do this? I think only to make the runners either start (if going on first move) or go back. So, I'd be inclined to, umm, view this suspiciously.

Balk.
I can't think of a reasonable excuse for him to do that except to disrupt a runner. I've seen balks called for similar mitt movements only a few times but each one was dead on. Deception or stupidity both get penalized on teh ball field.