Continuity mistake: Aquaman asks Mera what her name is while a strand of hair runs along his forehead. A shot later, it's gone.
Continuity mistake: When Aquaman watches the news talking about him and the submarine, his left hand swaps between being around the beer glass or not, depending on the shot.
Continuity mistake: After the submarine scene, Aquaman shows up on the pier. Check the wooden plank on the right, and you'll see that the puddle changes size three times between instant cuts.
Continuity mistake: In Italy, Mera fights against a guard, who crashes against a bottle cabinet, causing several bottles to fall. A shot later, the bottles are back in place.
Continuity mistake: Atlanna hugs Orb, her left hand over his shoulder. Then she places it on his chest and says, "Arthur saved me." An instant cut later, her hand is back on his shoulder.
Continuity mistake: After Tom suggests naming the baby Arthur, Atlanna holds his head. An instant cut later, her hand is along the baby's back.
Continuity mistake: Tom introduces himself after Atlanna does, who is holding her mug with both hands overlapped. An instant cut later, her hands are apart.
Continuity mistake: Mera takes her bracelet off, lays it on the street and smashes it. An instant later, it's in perfect shape. (01:30:37)
Continuity mistake: When the village is being destroyed, there's a bearded man with a green jacket who runs inside the church to take shelter. A couple of shots later, he is outside the church, running inside again.
Continuity mistake: Mera touches the stone where the Trident of Atlan was created with her left hand. From the opposite immediate angle, her right hand is now extended.
Continuity mistake: Mera's hair falls on her arm or not when she lies on the beach shore with the guard next to her.
Answer: You said it yourself some might have stumbled upon it, others could have used the map, which is why it was hidden in the first place.
How could they use the map when Arthur was the first to find it...quite frankly to stumble on it is impossible, you would have to randomly travel to the trench, go through the entire kingdom, see a tornado of death and decide to go in, get spat out and not eaten, get to shore, check behind a waterfall and follow it down...bit much?