Are you sure someone hasn't slipped a prawn or two into one of your heating vents?
maybe... there were prawns at the BBQ we were at
thing is... I thought the mrs was making a meal of it on the way home when she was in the back... then the little one woke up in the passenger seat and said 'phwoarrr... whats that smell of fish? smells like salmon..'
I was catching big whiffs of it occasionally