1 detective

2 to do detective work

1 tell-tale

2 informer (esp. to police)

3 whistleblower

4 grass

1 secret code

2 ciphertext

3 password

4 PIN

1 to explore

2 to probe

1 secret

2 classifier: 个 gè

1 inextricably linked (idiom)

2 inseparable

1 to poke one's head in and pry (idiom); to spy

2 nosy

1 to feel in one's pocket and take sth (idiom); as easy as pie

2 in the bag

1 to stick one's head out and look around (idiom)

1 to pluck a pearl from the black dragon (idiom, from Zhuangzi); fig. to pick out the salient points (from a tangled situation)

2 to see through to the nub