I don’t know if this is true in the UK but in the US McD’s locations are nearly all franchises owned by local business owners. So McDonald’s the corporation could reasonably be unaware that the 15 (say) locations in Dallas TX owned by Horatio Hornblower were all staffed by trafficked persons.
I’m not saying this would absolve McDonald’s of responsibility but it would address the “being unaware” part.
The comment is a reference to The Wizard of Oz, where three characters are walking through the woods talking about their fear of “lions and tigers and bears, oh my!”