I filled my airlock up to the fill line I came back and it was 1/2 inch lower. Where did the water go and does it matter?

I've came accross the same situation several times, I figured out, I would fill then insert it, doing that push's the lid down causing sucktion which sucked the water down, and of course evaparation, warm wort, cold climate= evap. other than that just get a cup, and put some more in it. hope that helps.
and GOOD LUCK!