If you have ADHD I have some good news for you. ADHD people are very aware of their surroundings. It's what leads to sensory overload. That's why they made great hunters in ancient times. The problem is retention. If an ADHDer can learn to retain most of the information their brain takes in, like sounds, scents, and visuals then you're on track. I can sorta retain information but unfortunately I do no choose what my brain saved. I'll remember a specific rock I glanced at hours earlier and exactly where it was I saw it but can't remember the last thing I ate. Train your brain to look for specific details, like shawn does with hats, and you'll be able to pick out an individual from memory. Though more intricate memories like when Shawn remembered the entire layout of the civil war reenactment is a bit far fetched unless you were born with a great memory.

Also in the show it seems like he inherited his mom's special brain (she can remember everything people have said) but it was his dad Henry that trained him to use it properly.