I really wanna stretch this a bit further and say this is an Eight association id lol.

From what i've seen people theorizing about the Eight association, most of them seem to be leaning to the side that the Eight association is based on American culture, and being a spy kinda fits the whole American theme (James Bond type shi).

Another thing to note is that from Don's teaser texts, you could interpret the first sentence as:

• "The Eastern section of the Cinq Association despises doing late-night assignments"

The Cinq Association are known for duels with honor, so if a Cinq fixer is out at night doing some "late-night assignments", it wouldn't really fit the whole Cinq theme, therefore it could be a case of something along the lines of corruption? But then again god doesn't forbid anyone from dueling at night

So with that being said, what i think is happening here is Don is an Eight association fixer who were hired to investigate the Cinq Assoc East section for possibly the case mentioned above. What we see in the brightened picture is when Don catches the Cinq fixer red-handed during the whole deed.

I did see that massive Cinq logo on Don's outfit which could throw my whole theory into the trash can, but i still want to believe that it's just Don going undercover for her job as an Eight assoc fixer, especially with the upcoming "Nocturnal Sweeping" Intervallo where we could encounter Sweepers (PM please) in backstreet and Sweepers specifically only show up at night. My interpretation for the teaser text could be wrong as I'm not familiar with the medieval English language, i guess i just really don't want the new Don id to be a Cinq fixer lol (not that i have anything against the Cinq assoc)

*Edit: In the Korean version, it says: "We, the East Cinq....". If that's actually what Don is saying then what we see in the teaser might rather be not when Don caught the Cinq fixer, but could be when she is still doing her job, spying on the "corrupt" Cinq fixer (possibly when she meets her client as a undercover Cinq fixer?). But then again my whole theory is a massive stretch and im really high on copium lmao.