As a kid, with my first detector in the mid 1970s, with a Whites 66TR : In a field I got one of those old state of California highway signs for Hwy. 101. Heavy glazed baked plastic enamel on steel sign. It must have been several feeet across. And was in a shape (clover-leaf?) that had long since given way, in the early 1960s (?) to a new motto/shape .
I propped the thing next to a tree , thinking "this will be a cool whatzit for my backyard" . Intended to pick it up on my way out. But then forgot to pick it up.
Fast forward to the 1990s, and I was eating in a restaurant where the proprietor had decorated the interior with all sorts of old signs. He was a collector of old auto signs (gas, road signs, cola, etc...). I described the sign that I had found 20+ yrs. earlier as a kid. He said that they would fetch $700 or more if in decent shape. Apparently a collectible. And by now (another 20 yrs. later) it is no doubt worth even more. I kick myself for not remembering to hike it out.