This guide shows the location of 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 (currently, as I write this guide, 15/02/2020).
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

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
By cayreet