Trust Stan?

When they weren't working to put the Mystery Shack back together after Gideon's hostile take-over, Dipper spent every spare moment studying the new Journal, mostly at night with a flashlight.

Mabel called over to him, "Hey, Dipping-sauce, don't you think it's about time we told Grunkle Stan about the Journals? After all, he saved us from going to jail."

"That's true," said Dipper. "And in this town kids are sent to adult prison. Look at what happened to Gideon – he's in the big house at age ten."

"So let's tell Grunkle Stan tomorrow," said Mabel. "We shouldn't keep secrets from him. Not big secrets, anyway. Little ones, like the secret ingredient of Mabel juice... that's okay."

"You have a point," said Dipper. "But here's a counterfactual: look at this journal page."

"Possible Hiding Places," Mabel read. "So?"

"Look at the building on the map," said Dipper. "It's the Mystery Shack."

"No it isn't," said Mabel. "There's no gift-shop wingy thing."

"That was an add-on," said Dipper. "Remember when we went back in time and saw the building in winter? The wing wasn't there yet."

"Okay, but it still might be some other building," said Mabel.

"The layout of the roads, the woods, everything... it has to be here," said Dipper, "Hey! This explains why Gideon wanted the Mystery Shack so much. It wasn't just to get an amusement park location or to ruin Stan. He wanted to search around here for hidden things, probably the other Journals."

"He already had two of them," said Mabel.

"Not when he blew up the safe, he didn't," said Dipper. "He only had this one, and he must have been looking for the other two."

"It makes sense, I guess," said Mabel. "What does this have to do with Grunkle Stan?"

"Grunkle Stan has owned this property for years. The Author wouldn't put all the secret hiding places close to the Shack unless he was working with Stan, or... if Stan himself was the Author! He could be!"

"Grunkle Stan isn't that good an artist," said Mabel. "Those illustrations are fantastic, and I know art."

"He's a good artist when he wants to be," said Dipper. "He did the best Benjamin Franklin pictures of any of us."

"You can't accuse him without proof," said Mabel. "It still may be some other building."

"I want to go around tomorrow to the hiding places marked on the map to look for signs of recent digging," said Dipper. "If my theory is correct, then Gideon would try to dig them up while he was in control of the property."

Sure enough, every single place marked on the map as a hiding place had been dug up, and many trees showed signs of the search for a hidden panel.

"I stumbled into one of these monitors hidden in a fake tree, one that was just like this page shows," said Dipper. "Security monitor/locator. System number three, patent pending."

"Is Grunkle Stan that much of an inventor?" asked Mabel.

"I don't know," said Dipper. "I think he's keeping a lot of secrets from us. We need to do some monitoring of our own and get evidence."

"What, are we going to stay awake in shifts and watch Grunkle Stan 24/7?" asked Mabel.

"There's something simple we can try first," said Dipper. "Some nights, when I wake up in the middle of the night, I hear someone moving around in the Gift Shop. We could secretly turn the security cameras in there on at night and see what we can find out."