They found two bodies in that bag, and those two bodies were missing people.After she took the witnesses' statements, she began to doubt her friend (Margaret) because she met all the characteristics
And (Julie) started watching her friend (Margaret) when they went to collect evidence and she found that (Margaret) had stolen criminal evidence, not to put it in the investigation bag, not to steal it with her, which increased Julie's suspicion of (Margaret), but because of (Julie's) extreme intelligence, she did not tell ( Margaret) or make her feel something to confirm suspicions and gather evidence
Julie, because she is very smart, placed a hidden eavesdropping camera in the middle of the button of Margaret's shirt so that everything could be imagined and everything would be listened to after a while.