Continuity mistake: When the turtles return April to the roof after erasing the pictures off her phone, she takes more pictures holding her phone horizontally. Then, at Eric Sachs manor when she shows him the pictures, they are shown to have been taken in vertical mode.
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.
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 ★