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: 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: Mera's hair falls on her arm or not when she lies on the beach shore with the guard next to her.
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: In Sicily, the lorry that brings Mera and Aquaman drives way past a terrace umbrella towards a street. When the shot changes, the lorry comes to a stop several meters behind, next to the terrace umbrella.
Continuity mistake: Before Mera eats the roses, two backpackers, one with a pink T-shirt, walk by and head towards a street. When the shot changes they've moved several meters back and are walking behind Mera again.
Continuity mistake: In Sicily, three women are having lunch by the florist stall, the one with sunglasses sitting in the middle. Two seconds later she's now sitting on the right side, and also the man behind her has disappeared.
Continuity mistake: In Sicily Mera eats the roses and puts her right hand to her mouth. From the opposite angle her arm is down.
Continuity mistake: When Atlanna is found by Thomas, the trident by her face changes position between shots.
Continuity mistake: At the bar, when the bullies ask Aquaman if he's the fish boy from TV, he faces to the right. Angle changes and he's now facing straight, only to repeat the movement.
Continuity mistake: When Aquaman enters the submarine and the door knocks the guards unconscious, its position and the gun on the floor's change between shots.
Continuity mistake: When the bullies are nagging Arthur at the aquarium and the shark appears, the distance between the bullies is inconsistent between shots.
Continuity mistake: At the aquarium, the first shot shows all of the kids behind the glass, with a tall guy on the far right. When the angle changes, there's a big gap with no kids and the tall guy is gone.
Continuity mistake: Right when the guards enter the home and the walls explode, Arthur swaps between having short curly hair to it being very long and straight.
Continuity mistake: When the family is with the kid, the father reading the paper and the wall explodes, Thomas jumps to the left and covers his head with both arms. From the opposite angle, only one arm is raised, the other one is extended and on top of the newspaper.
Continuity mistake: After Thomas, the baby and Atlanna have their picture taken, Thomas is reading a newspaper. The position of his left hand is inconsistent between shots.
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?