While those lines do indicate stress, stress can be in the traditional form of something intimidating the fish, or it could mean stress as in system stress: the fish may be showing the first signs of illness. Now, the fact that he is hiding so much suggests something is scaring him. People sometimes don't realize how terrifying minor things in the out-of-tank environment can be to a fish; something like a change in the surrounding furniture, a child who taps on the glass, a dog running past on the other side of the room, etc. can make bettas feel very exposed. Consider the fact that out in the wild, they'd be in the safe, murkey waters of a rice paddy with plenty of plant cover. In our homes, they are in a little glass tank exposed on all sides to the whole world. You might just have a shy little boy in need of adjustment.
Might I ask a few questions in regards to this fish?
- How long have you had him?
- Where did you get him?
- What size tank is he in?
- Is it filtered/heated?
- Did you recently do a water change?
- Do you use dechlorinator?
- What is the temperature inside the tank?
- Does he have a history of illness?
- Have you seen this behavior before, or is it a recent development? (this is very important; if your betta was usually bold and is suddenly shy, he could be feeling under the weather and is hiding because he doesn't want "predators" to see him in a weakened state)
- How much plant cover is there?
- Are there other bettas/animals within its vision range?
- Was he recently bred?
- What are your ammonia/pH/nitrate/nitrite readings?