"The term 'witness holes' generally refers to the small openings on the back or side-face of a handgun magazine allowing operators to visually confirm how many cartridges are loaded. Most modern semi-automatic handguns come standard with magazines featuring witness holes. It may also refer to the hole drilled in the top of a pistol barrel chamber so a user may visually confirm if there is a round chambered. This serves as a type of loaded-chamber indicator (LCI) but has no mechanical parts and does not allow for confirmation in low-light conditions when compared to a traditional LCI."
—SIG Sauer Glossary https://www.sigsauer.com/glossary/witness-holes/