I had to use a harness with my 3rd. and I really mean HAD to. At the time I had an 8 year old, 4 year old, he was 2, and a newborn. As soon as his feet hit the ground he was running, he could unlock any lock and open any door.
When I had to go grocery shopping and stuff I very carefully got all three other kids out of the car first, put the baby in the sling, had the 8 year old hold the 4 year old's hand, slipped the leash over my wrist while I unlocked his car seat, and then tried to grab his hand before he made it the length of the leash into the parking lot. Without the leash, he'd have been across the parking lot before I could get the car door shut.
He's just always been that way, he hits the ground running when he wakes up, and doesn't stop until he keels over at night. I even used the harness when we co-slept.
: I didn't start that until he'd gotten out of bed before 6 am and was across the street playing in the neighbor's front yard. I didn't even wake up and it scared the living crap out of me. I had 3 locks on the front door, and he'd opened them all, and he was only 27 months old.