Behind the crates in the corner on the bottom level. In the corner of the pool to the left when looking at the security door. In the men's bathroom stall closest to the pool.
http://www.seriousgmod.com/threads/...ecret-knife-location-video.25869/#post-230893 This thread shows a video about where and how to get the secret knife. Also, there are 5 total buttons that must be activated in sequence.