Ah you're right. It is more tricky than what I imagined.
Right if I drop my bag on the ground it should immediately be free for all and available for police search. But trash is considered "abandoned".
I didn't know about easement but it makes sense thanks for pointing that out.
In many US states streets were never formally deeded to the municipality when the lots were platted, and there's a long standing legal principle that the lot owner has a fee-simple interest to the middle of the street. (Washington included, not sure about Oregon) ie, it's not public property, it's private property with an easement dedicated to the public.