What are these symbols on the wall in New Mombasa?

I can't speak for its validity, but the wiki states that they are be related to the Engineers (aka Huragok).

Huragok are known to mark some areas of significance with mysterious glyphs, which can be found on various surfaces, walls, ceilings, floors, even in transmissions, often in irregular patterns. There are three known types of symbols:

• The "Y"-shaped ones often with glyphs similar to Forerunner symbols around them.
• The "O"-shaped ones sometimes with a symbol in them.
• Or simply in lines.

The symbols are likely related to the Huragok's Forerunner origin,[57] though it is known that they are a deviation to the Forerunner variants of the Huragok symbols.

The symbols were also conveyed by the species through Huragok sign language. Each symbol is believed to tell "stories", and some represent the shape of Forerunner starships.

This is along the same lines as a lot of theories that were running around the time the game was released:

Story-wise, it looks to me like the symbols were made by Engineers. The Engineers are exploring the city, doing research. I think they leave these symbols as markers or signs. They're cataloging what they find.

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There is almost no information regarding what these symbols mean. Though I've come to believe that they're there for no reason, just to intrigue the community into playing the game more to figure it out. There have been theories that the symbols were printed by the Engineers, and there is a special symbol that is marked near dead elites.

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