Its a birthmark that always appears on the foreheads of male members of his mothers family, it has nothing to do with his mother being a slave, after all since it only appears on male members she wouldn't have itThere is nothing reasonable about this. They did not have reasonable grounds.
See, a special birthmark is an old trope used in literature a lot. And it's usually to indicate royal or an otherwise "special" bloodline. But in slaves? Why would the slavers memorize birthmarks?
This is SO unlikely it strains credulity. Here is what you should do:
Skip the birthmark entirely. It's stupid, it doesn't work. Instead, have a mark applied to ALL slaves - a brand, a tattoo, something like that. This mark was hidden on the boy by his mother. BUT the mark also had something magical about it (in case of tempering), something the mother did not know about - and neither did his guards (more on them later) know about it. So a routine border check, which the guards thought would be no trouble, revealed the prince to be a slave, and he was captured.
You've already spoken below about a spell. Again, you don't need the birthmark.
Again, WAY too convoluted, and not believable either.
- This is WAY too much trouble for slavers to capture a young boy.
- Are they slavers or "the authority" (you spoke of "arrest" earlier)? If they are the authority, they don't need these shenanigans
- A naive 4 year old is not listened to by his guards in matters such as this. If there is violence (muggings etc) they will protect their charge, not spread out.
As for how did they realize by the birthmark that his mother is or was, by which I'm referring to the fact that they don't know she's dead but they know that because she escaped 17 years ago that theirs a good chance that she is dead, a escaped slave, which means that since they don't know his fathers a king they class him as a slave
Back to the subject of the birthmark I would guess that theirs a registry of birthmarks that can be used to identify escaped slaves and even though it couldn't be used to identify his mother, since only male members of her family get it, it could be used to identify a male descendant of hers, and in this case it was used to identify a male descendant of hers
You did say that him bearing a birthmark that looks a lot like the 1 that all male members of a family that has been entirely slaves for close to a millennium isn't reasonable grounds to detain him and find out if he's the child of a escaped slave
After all even though being a slave snatcher doesn't require a high INT, but having a high INT helps, they will all have A INT stat that's high enough for them to all to realize that since he's only 4 years old he can't be a escaped slave, but he could be the son of a escaped slave
To that I say that it seems like you've overlooked the fact that different countries have different opinions of what is reasonable grounds to detain someone because by the laws of his mothers homelands they did have reasonable grounds