This guide shows the location of all secrets.
Where To Find All Secrets
Basics
Unless otherwise stated, clicking four times on an object will unlock the secret. Click on the object the hand cursor is hovering over (sponge in one case).
Unfortunately, secrets don't keep, the game 'forgets' you have found them after a while.
There are a few times when one secret unlocks access to another and one case in which you need an object before you can unlock a secret, but I'll write that down where it's necessary.
Christmas Eve In Metropolis
No secrets in this one, so no entry here, either.
Black Magic Academy
Left side, close to where the level starts.
Close to the first secret, a little below and further right.
About the middle of the level.
To the right and above of the last one, to clear this one, move your mouse across the fogged area as long as you see the sponge without clicking, message here is 'cleaned' and not 'unlocked'.
Pirates Of The Southern Seas
Left-hand ship, upper left corner of the level.
Right-hand ship, pretty much in the middle.
The Good, The Bad And The Sexy
Here, almost all secrets are in one vertical line, starting with the UFO.
Towards the middle of the level, on the top.
Below and slightly to the right of the first one.
To the right of the second one.
Below the second one.
Slightly below the third one, click on the dog.
From Transylvania With Love
A little below the starting point.
You need to activate the first secret to be able to activate the second one.
On the right side of the level, bottom area by the graveyard
Erik The Red **** Saga
Top of the level, towards the middle.
Lust Island Adventures
Towards the middle of the level.
Below and to the right of the first one, by the submarine.
TV Studio Backstage
A little above the starting point.
Hardcore Gym Workout
Bottom on the left side.
You need to activate the first secret to get to this one.
Towards the bottom on the right side of the level.
You need to take the plush rat from the dumpster on the bottom of the level to the rat in the lab to activate the screen, then you can activate the secret.
You need to activate the prior secret to get to this one.
In A Black Hole Far Far Away
A little to the top of the starting position, on the left side of the level.
Below the first secret, at about half-height.
A little below the second secret.
Source: https://gameplay.tips/guides/6645-perky-little-things.html