Yea it happens.
Several of my relatives are grape growers here in Niagara, and they often find people in their vines near a roadway picking whatever they want.
That large a haul is not common, but happens.
Then there is me, out picking a couple hundred pounds each year, but with permission
I recall a number of vineyards plant sour tasting grapes around the perimeter of their properties in the hope that locals and thieves won't bother to progress their search beyond the first one or two rows.