When I was hunting for secrets I found a couple things...
In level 1 you can get over the left wall
In level 2 you can get over the left wall
SPOILERS: ALL SECRETS I FOUND:
In level 2 you can jump over the portal, if you fall down, you will see a message: You will not be freed from the facility
In level 3,going to the left immediatly results in you landing on a platform that says:There is no cake (Portal refference?)
In level 4, jumping over the gap instead of falling in it will result in you reading a very long text that tells you there are a lot of secrets.
That was all I could find! Is there anything I missed?