What the heck is this????

This is the back of my garage (project dos.) Look at the bottom right corner of the garage. It's hard to see in this picture, but the next picture is a closer one. What the hell is this thing??? I've been trying to figure this out since I moved to this house. It's a huge concrete slab with 3 big metal things sticking out of it. Somebody obviously put this here on purpose, in this exact spot, with exactly three of these metal things sticking out in a very exact pattern. Take a closer look at the picture below. The upper right metal thing actually has a chain and lock on it. I have got to know what this is before I die, or move, whichever comes first. We have all sorts of outlandish ideas about what this thing is. The best one so far is that it was used to tie people up and torture them. That seems unreasonable since there was a very non-private chain link fence here when we moved in and I'm sure somebody would have noticed, and hopefully reported it. Coworker suggested it could have been used to tie up a dog but I can't imagine somebody would install such a contraption to keep a dog tied up.

If anybody has ever seen one of these and knows what it is, PLEASE, PLEASE, help me solve this mystery.

In the meantime, garden buddy says I should put a big pot with plants on top of it and some small ones to hide the metal things.