Hmmm, what could this be?

There I was prepping for cardboard and compost to go down and I heard a metallic sound against my trowel. 

Dug down and found this ... it looks just like the L shaped bracket used to screw the edges of the weed suppressant down. But this one is vertical!

I've had to just put a stick in the ground to mark where it is as it's too deep in for me to dig it out.