The phrase "mysterious briefcase" refers to a small, portable bag or container that is secretive, puzzling, or unknown in its contents or purpose. Full definition
We meet Lucy in a tight bind; a (soon to be ex) boyfriend has forced her (via sneaky handcuff application) to deliver a mysterious briefcase to a man called Mr. Jang (Min - sik Choi) in a hotel in Taipei, where she's studying abroad. (waytooindie.com)
Inspired by [The Case], Extraction mode features two teams competing to capture a mysterious briefcase. (digitalchumps.com)
But when he finds a mysterious briefcase, it leads him to his father's former partner. (eclipsemagazine.com)