Some of the secrets are only visible or accesable with spectator.
For a few you need noclip. (They are designed to nag people who are looking for secrets with noclip)
And for 2 your need additional players to spot the secrets.
And of course there are secrets that are incredible hard to activate. :P

With 7 secrets, "The Dark Zone" has the most. lvlworld.com/review/id:2478
Followed by "House Of Chthon" with 5. lvlworld.com/review/id:2433

Which of your other maps have secrets?

Here is a little demo how to pass the rolling boulder:
sst13.de/secrets/13arena1.dm_68
Yo have to be fast, and it's good idea to duck at the last part.
But you can also do it with rocketjumps, quad or a mate pushing you... ect.

Maybe I should add demos for all the other secrets in my maps too?