Surprised I haven't seen anyone theorize this about Mark...

I think Mark's outtie has chosen to undergo reintegration before he returned to lumon. We are only seeing the start of it and iMark is gonna have no idea whats happening to him.

As someone else here pointed out, the mysterious man in the suit that appears behind Mark in the opening scene does look like him, and disappears when Mark turns around after sensing him - we see similar behavior from Petey.

In the moments we see Mark in the elevator, it seemed like his transitions were progressively... Weirder. Both based on how they were filmed and also based on Mark's reaction to the transitions, like they were different than what he's experienced previously.

Another redditor pointed out that the doctor responsible for Petey's reintegration said that he didn't follow the proper follow-up procedure, which is why he was having very severe reintegration sickness, and the likely "correct" way to make reintegration successful was to continue going to work to allow the severed and unsevered parts to mesh over time.

Finally, I just can't imagine that oMark would go back to Lumon without some sort of plan to find his wife, and reintegration seems like the best (if not only) way to do it. I'm sure we will get to see what's going on on the outside soon, but id bet that after his sister and Rickon tokd oMark what iMark said, and oMark told them what he learned from Petey and the doctor, Mark goes back to the DR to get reintegrated while his family covers for him (thus why Rickon is quoted praising Lumon in that article in the newspaper that Milkshake shows iMark).

Edit: Sorry if people HAVE been talking about this, I've spent an unhealthy amount scrolling this subreddit over the last 24hrs but easily could've missed a post or two 😄