There will not be the full 15 pieces of evidence on the first pass. If they have Programming Level 3 they can also hack into the computers, although the computers may not provide a piece of evidence. If your Sim roots around the boxes, they'll get a piece of evidence.
Around the lab will be a few computers and boxes of files. The door will be sealed and your Sim will need to gather some evidence to create a dossier on the subject. The next part of the mystery will go quickly and can be started immediately after checking out the door. Assuming you've talked to people and bought an item as well, examining the door will clear the first part of the StrangerVille Mystery Aspiration. A large door in the center of the first floor cannot be passed through, but you can interact with it to check it out. Travel to the lab (it's located in the northwest corner of StrangerVille), then check out the lab. One constant theme of the rumors is that a sealed door is in the Secret Lab. Buy any item you wish here (even a Bizarre Fruit if you really want to) to pass the objective. Note that if you buy and eat a Bizarre Fruit, there are some side effects that may make completing the story a bit more annoying than it needs to be. Once you're there, you can browse the wares and buy whatever you wish.
In the screenshot below, it's the small shed in near the bottom-left corner with an animal skull on the roof. It's not a separate lot you'll need to travel to the bar or the trailer park, then look for the little shop. This is located across the street from the bar and right next to the trailer park. Your next task will be to buy anything from the Curio Shop. The Sims will give you some information, but for now they're just rumors. You could even ask the bartender as well. Any five people will do you may want to head to the bar because several Sims will be guaranteed to stop by. It's likely to pop up on the pie menu even if you haven't introduced yourself yet. To get started, you'll need to talk to five people while within StrangerVille and use the special "Talk About StrangerVille" social interaction. The oddity isn't very prevalent at first, but it's enough for your Sim to take notice and want to investigate. When you first arrive in StrangerVille, a pop-up box will hint that there weird people and weirder plants all around StrangerVille. A Sim with advantageous Reward Traits or Energy Potions will make it smoother. The storyline is still possible to complete without living in StrangerVille, but your Sim would need to make frequent trips out there. Any lot will do, whether it's the poorer or richer part of town. The storyline is easier to complete if you move your Sim into the StrangerVille World because of how often you'll need to interact specifically with the people of StrangerVille. Skül Thünderthrüst has the Hot-Headed, Childish, and Paranoid traits, none of which help him at all. However, both requirements are easy enough to attain even without Genius or Ambitious. At a certain part of the story you'll need to be at Programming Level 3 at another, being at least Level 5 in the Military career track can help bypass a requirement.
Some Traits you may want consider if you wish to blitz through the storyline are Genius and Ambitious. Remember that you can change any Sim's Aspiration at any time and retain progress you've made for their current one. For the purposes of this walkthrough, we've created a new Sim named Skül Thünderthrüst and given him the StrangerVille Mystery Aspiration. Any Sim can go through the StrangerVille storyline, but a new Aspiration called StrangerVille Mystery will help guide Sims along.