In the beginning we see that Yan was a stundent of Sam and a college of Lau, so both knew, that he was with the police and got kicked out even though he was a really good police student. And now he is with Sam and has a different name than back then ... the kind a gives it away : ).
Both of them, especially Lau, are no idiots. They actually should have know that Yan was the mole.
For Lau to suspect Yan I think it depends mainly on how close they were at the police academy and the amount of time that passed before Yan joined Sam's crew. Keep in mind that only one officer (who subsequently died) and Wong knew that Yan was undercover after the records of him at the police academy were wiped. After being kicked out Yan and Lau crossed paths twice, first when Lau was taking Yan's fingerprints and second when they met again in the electronics store. It's unclear whether they recognized eachother the first time but it's pretty obvious that they more or less forgot about eachother afterwards. Also In the second film it's revealed that Yan is related to triad members so Sam probably didn't have much reason to suspect him.
share