We're talking about a creature who just spent a minute
incapacitated on a demiplane. Is that right? Let me address a few issues that appear in your post before I tell you how I'd rule.
I know, I know, a surprised creature is only getting an extra beating before the first round action...
Actually, a surprised creature doesn't get to take actions or move on its turn
in the first round, not
before the first round. However, once the creature's turn in the first round has passed, it does get to use a reaction if it has one available.
But could it be that you would rule that a monster returning from "Banishment" after leaving the encounter for, maybe, a minute, that this creature could be caught in another surpriseround?
There is no separate "surprise round". Surprise is effective in the first round of a combat encounter only, so if the encounter from which the creature was banished is still in progress when it returns then no surprise is possible. If combat has otherwise ended, however, I could see the creature's enemies attempting to hide from it in order to surprise it when it returns.
I just think that a monster who knows nothing of "banishmentspells" should be quite surprised comming back from a trp to some other plane? or what is your opinion on the subject?
Since it was incapacitated while on the demiplane, the creature may be somewhat disoriented when it returns, but not enough to impose surprise. It may not be aware that any time has passed at all, and it will most likely be expecting trouble when it realizes it is back in the same place. Surprise only applies when all threats to the creature are hidden from it, and it is only determined at the
beginning of an encounter, i.e.
before initiative is rolled. Therefore, if combat is still going on when the creature returns then surprising it will be impossible. If, on the other hand, things have quieted down by the time it comes back, and it is unaware of the presence of its enemies, then I would feel alright with it being surprised going into any combat that would follow.