Wonder what this was?

It's what you think it is I'll bet. 

I had a huge elm removed a few years ago.  When the stump was being ground we found a 2" steel fence post bottom the tree had grown around.  The post had apparently been cut off at ground level before the tree eventually enveloped it.  The guy running the grinder said he'd ran into all sorts of stuff over the years.

@eddycreek

Here's another "suprise"  

Working today in a city subdivision from the 1980s.  A cedar tree of about 18 inch diameter appears to be dead center of where a 1/2" iron bar was set for a lot corner.  Get a nice iron signal somewhat evenly around the base.