Seriously, where are they? They're just two random lines.
So then, here are some possible reasons as to where they are...
1. Misty thought it would be funny to glue his eyes shut while he slept.
2. He stuck his face to the screen watching some weird Japanese show that gives people seizures and his eyes exploded.
3. He's on drugs and doesn't have enough strength to pry them open. Plus, he doesn't want to stop watching the swirling colours in the darkness.
4. Team Rocket put a homing device in each of his eyes. This is how they can always find the twerps.
5. He had no eyes to begin with, but people thought of Pokemon as a Satan-worshipping show because a guy was walking around with no eyes. The animators then drew two lines to represent his loss of eyes.
6. Ash thought they were a Pokemon so threw a PokeBall at Brock's face and caught his eyes.
7. He was making faces at people when the wind changed.
8. Someone poked them out because they were scary.
9. His eyes revolted against him, jumped out and migrated to somewhere warmer.
10. One day, he thought to himself, "Hmm, I wonder what would happen if I poured this deadly chemical into my eyes- GAAAAAH!!!!"
OMG WHAT THE HELL?!