I don't think that they confiscated his passport because they considered him a flight risk. He certainly couldn't take his SS entourage with him, so he probably has no plans to use it for anything. It may be that they found it among the documents that they confiscated. If so, they'll probably return the passport, if they haven't already. They don't normally confiscate them until charges are ready to be brought, and that is not yet the case.
I would certainly consider Trump a flight risk.
The only reason he might not be a flight risk is his ego. I’m sure he could find refuge in any of several countries.
Yes, there could be some complications with his SS detail—but there is speculation that at least some of his detail is( or was) loyal to Trump rather than to the US or the SS. I don’t know how much his detail changed when he left office. I do remember that he secured an unusually large number of agents to protect more family members than generally necessary.
From what I understand his passports have been returned, but in the case of his Secret Service detail I think there's a few more wrinkles. If he were going to flee the country to avoid federal law enforcement, he'd have to ditch the detail, as they ARE federal law enforcement. They do more than just protect Presidents, after all. While he may have a few that are loyal enough to - say - delete phone messages and cover up his little tantrums, I doubt that they'd be loyal enough to flee with him to another country. That would make them fugitives as well. It is also worth noting that most people are loyal to Trump
to a point. We know about his shady dealings mostly because people who were loyal to him finally had enough...some have even gone so far as testifying in front of Congress. Yeah, he's got Rudy and Stevie and an inner circle that are still on board, but would Secret Service agents be so loyal that they'd abandon their families and country to go live in wherever he flees to? Hmm...
As far as Copernicus' mention of his lawyer Jay Bratt, that brings up a whole other can 'o worms. Several cans, actually.
Apparently some of the classified material was Top Secret SCI that you can't even look at unless you have that level of clearance and it has to be in a secure, designated location. It can't just be thrown into a box and shipped via UPS to Florida. Not only did someone (obviously other than Trump) have to gather those documents, but they would have to be transported by an authorized government courier with the chain of possession thoroughly documented. You can't just walk into the room, say "the President wants to see the report on the alien spacecraft at Area 51" and walk out the door. Who removed the documents? Who moved them to the White House? How was it moved to Mar A Largo and who had access during transport? There could be any number of people who were involved in the conspiracy to take these documents to a storeroom in Trump's basement, and I use the word "conspiracy" on purpose, because I'm pretty well certain the FBI and DOJ are using that word, too. Anyone who touched that material along it's journey from a secure facility to that storeroom could be facing some serious federal criminal charges.
I'm also fairly well certain that the FBI asked all these questions and more when they were investigating Hillary's Buttery Mails.