This small and simple guide will help you quickly complete the “Hiding in plain sight” achievement”
First is located under the window next to the computer

The second one is located behind the monitor

The third one is under the shelf that is closest to the entrance

The fourth is in a small room under the electrical panel

The fifth one is under the stairs

The sixth is behind the ventilation system which is under the stairs

The seventh is located under the shelf where the palettes are located

The eighth is on a fan under the ceiling

The ninth is next to this fan on the hood

For the next two, it is necessary to unblock the premises with parcels
The tenth is pasted behind the monitor

The eleventh one is in the trash on the gate on the back side

Thanks to VeeXell for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.