I missed this BBC News report in August 2017, it is fascinating reading and proves the KSA probably thought "We've done thsi before, who will care this time?". It opens with:
Between 2015 and 2017, three Saudi princes living in Europe disappeared. All were critical of the Saudi government - and there is evidence that all were abducted and flown back to Saudi Arabia… where nothing further has been heard from them.
There is an update:
Update, October 2018: Two months after this story was published the former head of Saudi intelligence, Prince Turki al-Fasial, confirmed that all three missing princes were back in Saudi Arabia. Prince Khaled bin Farhan, meanwhile, says he has heard from sources inside the country that Prince Turki bin Bandar and Saud bin Saif al-Nasr have both been executed. This could not be independently confirmed.