Revealing mistake: When April first takes the photograph of the symbol, she actually has the camera in selfie mode, you can see her face on the phone.
Continuity mistake: After the turtles, April and Vernon escape from the Sacks compound in the 18 wheeler, the truck then get stuck in the snow. When they realize they start to slide backward down the hill, Vernon guns it. But then they show the wheels spinning on the Freightliner Argosy Cabover with a single axle. Next shot goes to inside the cab and then back to outside the truck, and it's back to a dual axle truck.
Answer: Sacks told April that he was raised in Japan by a local sensei, and that he shared the lessons he learnt with her father. Splinter would have overheard Sacks mention his master at the lab, as well as when O'Neil discovered what he was up to with Project Renaissance and his connection to Shredder and the Foot Clan.
Sierra1 ★