First time it's an unarmed nobleman he does this to, he's likely as not both out of a job and blacklisted. If not getting shot by a firing squad following a show trial for murder by deception, or by same said marines "resisting arrest" for fraudulent emergency alerts.
At a distance, it's a vacc suit. Sure, it's an armored vacc suit, but it's still, at the end of the day, a vac suit, and starports are canonically LL3. Anything short of PGMP's and explosives is legal for open carry.
Further, Battledress is ideal for engineers - highly protective against many forms of engineering mishap, plus increased carrying capacity.
And its canonical that people can wear armor, even combat armor, in starports pretty much unhindered. And carry firearms, and blades, and such - provided they aren't carrying them at the ready.