[magnusmundus]

Literally every check-in form I've been through had some required checkboxes to declare that you've packed your luggage yourself, have complete awareness of what's in there, and claim responsibility for such items. There are also countless, very visible, posters in airports saying so. Your paper trail might be useful to law enforcement in prosecuting the people who handed you the no-no stuff, but it doesn't absolve you of any responsibility.

Anecdotally, I once had an outrageously difficult time explaining to a German customs officer that the bag I placed in the scanner had items belonging to both my sister and myself. It didn't help that we were travelling together, and she was also there to back my statements up. He simply would not accept the fact that there was an electronic device (e-book) that belonged to her, in a bag that belonged to me, which we forgot to take out before the scanner.