In the bathroom of our new house, the grout between our tiles in the shower room seem to have occasional "bubbles" or gaps and I'm concerned that it might allow water in, especially since this is inside the cubicle and the shower blasts water right onto this area. The are dozens of these little holes all over the tiled area, but I've taken a photo and highlighted just a few as an example:

I'm guessing that this is a result of poor workmanship, but can this easily be repaired without re-grouting the whole wall, or am I worrying about nothing?