Strange circular holes in silicone.

[Gronith79]

I have refurbished this old tank 3 years ago. I am now seeing strange circular holes in the silicone. I has go me worried about the integrity as a whole. I have 2 fish in the tank and 3 shrimp currently. It is a 40 gallon hex tank. Any idea what is causing this?

 

[Tank20]

It appears they might be small air bubbles that formed in the silicone.
If you are referring to where the 2 panes meet.