I ran into a situation like this one time. I was scrapping at a forclosed property as the previous homeowner was still in the process of moving out.

The bank had possession of the house, but the previous owner had 30 days or so to move his things out, and there was already a new lease agreement for some charitable organization to set up a year long fund raiser tied to the historical value of the property. This is where it got complicated:

The homeowner was letting me scrap his things, the bank was not, and the new leasee had some issues with what belonged to the bank and what belonged to the homeowner. I ended up having to hang out all day on the property and waiting for all three people to show up before i could even take anything off the site, and even after all was said and done, i had to return about 150# of old broken lead gutters that they all agreed i could take in the first place. it was a mess, and im not sure i would do it again.

However, in your case it sounds like written permission from a bank representative would be enough to scrap junk found outside of any buildings or buried in the ground. if all, it takes is a few phone calls to get it, go for it! scrap the planet