Who's fault is it?

I've always taken a two prong approach with my kids in dealing with things they can't have or touch, etc.
First, tell them no and take it away.
Second, put it where they can't get it.
'Cause really, little kids are going to touch if they can. I've never really had a problem with any of my kids getting into stuff UNTIL Sammy. Sam has gotten into everything from the day he could move around. He learned how to defeat every kind of child safety latch and lock. The biggest "oh no" that Joe and Tom ever did was dump the dog food on the floor or into the dog water.
It only takes Sam about 30 seconds to find something to get into. Even when I do a visual sweep of a room and think everything is okay, he'll dig through a closet to find something. (which is why we've had to call poison control twice!)
You name it, he's gotten into it: markers (too many times to number!), stamp cleaner (3 times he's dumped in on my craft chair), make-up, nail polish, nail polish remover (poison control), baby oil (poison control), cleaning products, latex paint (still no sure how he got the lids of the can, though he's done it twice with two different cans), food, and water. There's more but I am tried of naming them. He's eaten candles, painted floors, superglued books, wood-glued the table, covered the dishwasher with soap, and more.
Sometimes, I feel like it's a game to him, but to me, it's just plain frustrating. I frequently get told that I just shouldn't leave him alone. Most of the time this happens, I don't consider him alone. He'll be playing on the living room floor and I will go to the bathroom. I'm gone less than 2 minutes and I come back to find white paint all over the floor.
Today, I was cleaning the bathroom and had already put away the bathroom cleaner. Sam was playing in his room with Tom. I was cleaning the mirror with glass cleaner when the phone rang. I answered it in the kitchen because that's the closest phone. No, it is not a cordless. I had to take the call as it was one of the many insurance people we've had to talk to over the last week. I was only on the phone for less than 5 minutes. I had to give the lady Jay's work number and then take her number and name down. Done. I had set the glass cleaner on my dresser as I had walked through my room to answer the phone. When I returned to my room, Sam was back in his room with Tom, door shut. However, the 3/4 full glass cleaner bottle was empty and my bed was soaked. I had to then strip the bed before it got into the mattress (too late!) and start the process of washing all of our bed laundry. 1 load of sheets, 1 load with the plush mattress pad, and 1 load for the blankets.
At what point should I stop blaming myself for leaving things out? At what point will Jay stop getting irritated with me for leaving things out? When does it become all Sam's fault?
Sam did get a time out for his behavior, but it never seems to make a difference.