The secrets hold armor, medkits, weapons and other useful stuff.
Download the space combat map for Half-Life
Secret 1
Each of the large platforms has 2 secrets, each holding 4 medkits and armor.
On the map, the way to the secret is marked with an arrow.
Secret 2
The middle platform of each arena has 4 recesses. If you aim at the floor and press E, a door will open, and inside you will find a medkit, armor, a Hornet Gun and MP5 grenades.
Secret 3
The next secrets are not that easy to find. They are not about hidden doors. Instead, you fall into a certain spot and then get teleported somewhere else. There are 3 such spots in total. Here is the first one; it is located on arena 01 (the arena number is written on the buildings, and there are two of them in total).
Fly onto the building so that the platform with the gauss is on your right. Land carefully so you don't end up in the teleport to the other arena.
After that, stand on the right corner and carefully drop down along the corner. Don't jump too far out, because you can easily miss.
If you did everything right, you will appear in a secret room with a gauss, medkits, armor, a long jump module and the signature of the map's author (that is, me =)
