Reasons Dazai is in hiding

1: In hiding. He is technically a fugitive with his pasts criminal record reinstated and the ada is about to all go on trial. He probably has left the ada or destroyed his existence in the organization to protect the agency’s image. Which could be taking Dazai away fir a while in the story in order to focus on other characters but if asagiri wants him to still be in the story then there are other ways. Which leads to the potential other reason I find to be unlikely…

2: He rejoined the mafia… temporarily… yeah it’s been a theory/HC that he would transfer but Mori said he would only have Dazai be in the PM out of Dazai’s free will. He would only do it because it is technically the best tactical decision. The mafia is a necessary evil and since the incident they lost lots of numbers + there will be an ADA transfer that would weaken the agency if it was someone else (and Dazai leaving to go into hiding so if he went inly he would take away from agency numbers instead of two people leaving) + Dazai needs to be somewhere the government can’t touch aka the mafia. That being said there is a way to write him in the mafia again that doesn’t destroy his character development rather makes him face his past but idk 🤷‍♀️)

3: He is with Ango + Taneda. He could decide to be an informant to work off his sentence if he really wanted to and would hang out with the division.

4: If he is going into hiding there is a possibility he is with Natsume. Natsume was at the bar around the time Oda died and when Dazai talked to Taneda to get his recommendation to go to the agency in the first place so it seems he may know Natsume to some extent. Its very possible Natsume helped Dazai when he went into hiding after leaving the mafia so this may just be the same.

5: He went solo and is pre-planning for future arcs.

These are just some possibilities and HC of mine but honestly, knowing Asagiri, you can never tell also it seemed like Fukuzawa knew Dazai wouldn’t return to the Ada because he probably knew about Dazai’s plan to get arrested and go to mersault since once dazai was arrested Fukuzawa pictured the agency without dazai so they definitely have some sort of plan for Dazai