Look, it's a secret! achievement in Serious Sam Collection

I actually got this when playing in co-op as I started playing the first couple levels in single player then once I got to the 4th level I decided to host a game in co-op so I selected the 4th level and continued it and then 2 people joined and I found a couple of secrets a little later on and the achievement popped while I was in co-op.

I should also mention I had the unlimited ammo on and the difficulty set to easy.