But, who goes on a picnic in February? nobody...
Not true.
You're not from Cooper Country, are you Fly? Picnicking in February may not be commonplace, but it is not unusual. We have two seasons in Cooper Country: Hot, dry and delightful in July and August. Cold, cloudy, rainy, drippy, dismal and cool from September until June, which is often called Junuary, to distinguish it from the more centrally placed month that starts the new year.
It's never really cold in our rainy season. It's just mucky and yucky. Hence, we are liable to do about anything at anytime. Lots of folks wear T-shirts and shorts year 'round, so roasting weenies on a cool Sunday afternoon is no big deal.
Case in point - I BBQ year round.
I grew up in the Vancouver BC area and had friends throughout Western Wa in my younger days, so I am familiar with the PNW climate. Our climate here is very similar and it would be extremely rare to go to the "beach" for a picnic in February with a young family. I have never heard of it and I have lived next to the water/ocean my entire life. Home BBQing year round is common but that is different from a "beach" picnic.
Taking a young family to a "beach" in 40 degree weather in early February for a picnic is not normal.
Another thing that is odd, they put the found money into a plastic hot dog bag but claimed to clear the area to start a fire for warmth. So, did they have hot dogs, didn't they need to cook them? If so, Why didn't they start a fire to cook the hot dogs?
And, if they did a search after they found the money, they probably did it by hand so it would have been very superficial. They left within two hours, so why didn't Dwayne and David return with shovels to do a more thorough search? They were young financially struggling families, you'd expect them to make sure there was no more, but they never returned. If your brother came home with bundles of cash from under the sand wouldn't you want to got look for more,,
As for motives, I gave a few examples,,
They may have found the money on private property which would put any claim in jeopardy.
They may have stolen it or received it from another party.
They may have outstanding debts and wanted to monetize the find through the children.
Right behind U-Cart Concrete where Dwayne worked is what appears to be about a 20 acre lot that is shrubs and a large marshy wetland with a permanent pond. Looking at images over the years it seems to have seasonal flooding and drying. This is under v23.