The locked briefcase on the picnic table has two combination dials and a tidy little stash behind them. You don't need to go hunting for the clues — the code is fixed in every world. Here it is, plus where the case is, what's inside, and where the game hides the clues if you want to earn it honestly.
The briefcase sits on the north seat of the picnic table, in the Picnic Table Area. It's one of the first set-piece puzzles you'll stumble onto, and it's worth opening early for the Raw Science alone.
Walk up, interact, and you'll see two combination locks side by side. Each has its own three-digit dial and a button to confirm. Set the left to 137 and the right to 514, press each button, and the latches release.
Because the code is fixed you can skip this entirely — but if you'd rather solve it the intended way, both numbers are scrawled out in the world.
Written in pen at the bottom of the newspaper / crossword on the right side of the picnic tabletop — right next to the case itself.
On a ripped newspaper corner atop the podium / lectern in the north of the Ceremony region, just south of the Dr. Tully statue. Reaching it takes a little parkour and ties into the "ORC Receivers" objective.
Small case, solid haul for the early game:
Knocked out the briefcase? Here's what else is worth looking up in the backyard.