Flooring Tile - small hairline cracks in grout

I finished my backsplash with 4" tumbled marble with 1/8 spacing. However as it dried a few hairline cracks have formed. Mostly where the corners meet. My guess is the tiles with round corners have a larger gap than the 1/8. The home depot guy told me I would be fine with unsanded grout. Looking back I guess I should have used the sanded. My question is if there is an easy way to fix these small hairline cracks? My guess would be to wait till it is good and dry and then mix a little more grout up and try and get it worked into the small cracks. Will this work? Any thoughts would be appreiciated. The cracks are not really noticable unless you get up very close to the tiles but it would be nice to have them fixed.

Sounds like you have the plan, procede with that.