The item in question was the subject of much scrutiny in the events leading up to a shocking bomb attack on the auditorium where Home Secretary Julia Montague (Keeley Hawes) was delivering a controversial speech.
It was given to new PR advisor Tahir Mahmood by Julia's slimy employee, Rob Macdonald, and scrutinised by bodyguard David Budd (Richard Madden) outside the auditorium – but we never actually saw what was in it, and many fans on Twitter believe that it was carrying the explosives.
"We thought long and hard whether to show you what was in the briefcase," Mercurio tweeted on Monday morning, as Twitter went wild with speculation. "And decided not to."
It's a teasing remark from the writer, as by signalling out the briefcase, he's suggesting that viewers are on the right track - without actually giving us any concrete info about its significance. Guess we'll have to wait until Sunday to find out...
Bodyguard resumes on Sunday at 9pm on BBC1