How do you determine obstruction?

[DBull]

After being out of umpiring for over ten years, I recently worked the bases in a game under FED rules.

This situation came up several times during the game:

A runner (from the visiting team) would be on 2nd base, the ball would be in possession of the catcher who would be faking a throw to second, or the pitcher. The runner would be standing about four or five steps off the base.
While this impasse was going on the short stop would move up right next to the runner, in the base path, and stand there.

The runner was not attempting to go to third, so I saw no obstruction; but what if the runner had made a move toward third, would this then have been obstruction?

This move by the short stop seemed to be a regular move that she used all the time.