My eldest has ADHD. She is an absolute dream child, really well behaved, rarely had tantrums. Even now she is very emotionally astute, cares for others. She slept for hours, is quiet and loves her own company. She is bright and creative. She actually has very good impulse control and is a real rule follower.
She has inattentive type, possibly combined ADHD.
We first noticed signs about 18 months old, she went for an EEG for absence seizures, the consultant we saw suggested then that we keep an eye on her for attention issues because she wasn't seizing, she was just going off in her own world.
At school she doesn't cause any trouble, but what she does do is not pay any attention. She gets over looked, drifts off into fairyland and has to really work at concentrating before she can do anything else.
She would still have the same issues if she was the hyperactive subtype, however you wouldn't be able to look past the "naughty", "disobedient", "defiant", "bouncing off the walls" issues to really see what lies inside.
With my own daughter we are currently celebrating as she's gone from needing constant 1-2-1 helping her to being able to concentrate for 5 minutes at a time. That's so mega it's unreal.
Definitely not an attachment issue either, or a lack of interest. My entire energy revolves around her and making sure she's set up just right with love, sensory feedback, sport activities, food, sleep, downtime. It's exhausting.