Some people just have more visible veins. I know mine are easily visible especially when my body fat drops slightly below my normal range (no clue as to the %). Typically the more muscularly defined you are the more your veins will appear.

"When dieting your veins disappear between meals and appear after meals. Top bodybuilders note that ingesting some simple sugars before a competition brings out their vascularity."

from: http://www.abcbodybuilding.com/magazine/howtoanalysizeyourself2.htm

If you look into any in depth bodybuilding guide they always talk about vascularity (the veins appearing). Take it as a good sign.