Body Filler in Cab Corners

[bladerunner]

So I have know for awhile there is some bondo in my cab corners, but I don't know how much.  If you knock on it, the sound is very dull, and not metallic like knocking on the rest of the body. Also, a refrigerator magnet will not stick to it.  I bought the truck cheap, so I figured no big deal.  The funny thing is I can see the sticker of the replacement cab corner on the inside of the cab corner, and the metal there is clean and new.  So is this simply a rust hole filled in with bondo, or did the previous owner just cake on a thick layer of body filler over a good cab corner?

I have thought about those gauges that can detect the thickness of paint and check for bondo, but I don't know how well they work. I might be able to have a body guy use one to check.

[Engineer]

Sounds like the mud was used to cover a bad weld/grind job.

[bladerunner]

Did not think of that.  I noticed that there was a lot of tearing around the rockers, probably from when the previous owner removed the old panels. Perhaps something similar happened here. Without cutting out the corners there is no way to tell I guess. I suppose one day when I replace the cab corners I will find out.