On December 6th, 2018 an Achievement was added to Sokobond: Secret Symbols. The objective is to collect 13 Sigils, each of which is obtained by solving a level with hydrogen.

The Sigils are part of the Eye ARG[wiki.gamedetectives.net], which spans multiple games, each containing a piece of a map. Sokobond's part was solved before the connection to the ARG was even known by reddit user AmitaiGaunt. This guide is based on his work. You can watch a video with solutions to twelve levels here and have a look at the final molecules here.

I'm using the layout Yiasmat used in their amazing guide, because I think it is a great way to show the information concisely.
The big arrow on the left denotes the direction you should start in. Once you hit another arrow, you switch direction, repeat this until you hit the circle. The circle is the starting position for the next image. Pictures are ordered left to right, top to bottom.

There are 13 Sigils in total, each one obtained by solving a level by forming one or more Hydrogen molecules. You can't miss them in the first two levels, but for the other ones you have to go out of your way, sometimes by quite a lot, to obtain them.
You will find the Sigils in the following levels:

• Multiple
  
• Hydrogen Machine
  
• Water, Water
  
• Aye
  
• Double-Bleached
  
• Ocean
  
• Corners
  
• Cat
  
• Halves
  
• Playful
  
• The Monster
  
• Grater
  
• Much Methane

Thank you for visiting my guide, I hope it was helpful.

Thank you to

• AmitaiGaunt for finding the Sigils.
  
• Yiasmat for their Full Completion Guide.