As it turned out, it was quite the coincidence.
Ni Guangli had a sudden stomachache right before boarding. Seeing there was still time and the restroom wasn't far, he went. By the time he rushed back to the gate, all the other passengers were already on board.
When the police arrested him, no one saw except for a few staff members.
Furthermore, this type of case required secrecy. The flight crew received the news that Ni Guangli couldn't board and naturally didn't dare to spread it around, so they just carried on with their flight duties.
Now that the entire flight crew had perished, everyone at Toutou Airport who knew about the situation had turned over their phones and was quietly working on catching drug traffickers.
Ni Guangli was arrested because something was found in his bag. This was done both to prevent any news from leaking and to protect him, so not a whisper of it got out.
