I don't think the gold bar was left out, at least I can't imagine anybody being that stupid. I imagine Beverly told the cleaning lady to take down some old boxes from the attic or clean out an old desk or other piece of furniture before trowing it out (as she was renovating the place) and that's where the cleaning lady found the gold hidden amongst other old stuff.
But yea I totally agree with you that this case is about the gold, that's the motive. Very likely they tried to sell it to some shady person(s) and things went south when Beverly met with them.
A friend of mine was killed by her estate agent when she tried to sell one of her apartments. He tried at knife point to make her sign the property over to him, when she refused he killed her (he beheaded her and took the head with him, sick bastard). She was also in her 70s, fit but elderly, and not a match for this young guy.
Elderly people should not meet alone with strangers to make deals that involve large amounts of money. Sad but true

I wish she had just gone to the bank with that damn gold bar!
Pretty sure we will hear at some point that both were found dead somewhere remote, shot most likely. And the gold will never be found.
Sorry for all the bleakness

But I have a hard time keeping up hope for these two.
ETA: Wikipedia page of my friend who was murdered
https://en.wikipedia.org/wiki/Gudrun_Corvinus