Yes it has been done
No there is nothing behind it
Yes it just flops open if you cut it
Yes it is pointless to do it or talk about it

 

Looking at the pictures, it seems like you might have the right idea. I'll check my copy in a bit to see for myself if it's from the tool design or if it's from stress.
Also, flash is excess plastic around the parting line of the part (where the mold halves come together) or at the vents in the tool. Where it comes off the runner is a gate scar or witness mark.

Edit:
Those are stress marks. I couldn't tell much from the pictures that posted, but in person it's much clearer. The gate is on top of the leg piece, to the side of the knee:

So that's not the cause of the mark.

The parting line of the mold goes around the part along the yellow line in this image:

Here's a couple pictures of the mark and an edited one highlighting it in white and the parting line in yellow:


You can see that the mark isn't along the parting edge, so it's not flash.

It's stress whitening, like @TFXProtector said. It's right along the area that flexes the most when assembling the leg, so it makes sense that a stress mark would be located there. It's also in an area that has higher internal stresses from molding. Here's a picture showing how the plastic fills the part overlaid in purple:

The areas where the flow is more radial/circular will warp more and have higher internal stresses.

As long as you don't take the leg apart (like I did for these pictures), the stress mark shouldn't get much worse. I'd be more worried about the side of the rectangular cutout opposite the gate, where the plastic welds back together after flowing around the shape of the cutout. That's going to be a weak spot.

 

This figure ROCKS. I was initially only picking it up out of completionist sake but in-hand-- I like it a lot. Great sculpt, nice heft and great articulation with very good joints. Is the transformation dumb? Yes but that's just inherent in the design.

Let's get the other two Hasbro!

 

I think it's a good question. Personally, I'm a little bummed it wasn't made a feature as I enjoy the idea of The Insecticons being vehicles as was their original intent pre-Transformers. Maybe it'll see a minor retool to allow for the cockpit along with some wheels for the Diaclone repaints.