1. After entering the tunnels and coming to the first water hole it will be under water. before getting to the vortigaunt that is saving Alyx.
2. Right after leaving the vortigaunts that are saving Alyx and going into the tunnel with the main vortigaunt to get the extract. it will be on the wall.
3&4. After fighting the first wave of acid-lions there will be one and after fighting a second wave of Acid-Lions there will be another.
5. Later on down the same tunnel there will be a weapons cashe above head.
6-9. Will all be in the elevator room were you need to attach the gear (two above head and two on the walls).