Solving Puzzle #5

I tried yesterday a bit (since nobody solved it), but cannot find a method to use the triangles. Anyone?

Here the obvious letter I see:

I was thinking, the words “deja vu” are strange and that you have to something you’ve been doing at another puzzle. The only one making sense would be number 1. So I tried to get the first answer with consulting the solution. 0% chance. I tried reverse engineer it. Even worse.

So I can not addopt the way for #1 to solve #5 at all.

Is this related to International maritime signal flags - Wikipedia

each flag means a letter, or a phrase. But I could not figure it out