We've seen marks inside of the auction houses though. Most recognizably Do Flamingo's smiley. Not much of a stretch to think that people do brand slaves. So even if it's not one of the more likely theories it could be possible seeing as there isn''t anything predominant that seems likely. So I think it's a bit much to say that his theory sucks.
Just because we see a symbol on a wall this leads to the logical next step that this same symbol is branded unto people? Symbol on a wall does not lead to the next step that very same symbol is branded on all slaves. In fact that's a horrible jump in logic. and yeah that makes a theory "suck." I think people are bringing their other Manga/Movie experiences where they've seen slaves branded. We've seen like 20~30+ slaves in OP… never have we seen one branded... never has their been any mentioning of marking or branding... we have no indication that Hancock has ever been a slave...