I used one once. We were buying a leasehold flat with a supposedly absentee freeholder - it was thought that he'd returned to his native country, but it caused all sorts of problems with the purchase.
In order to later buy the freehold, we had to show that we'd made every effort to find him.
So I employed a firm of PIs, fully expecting them to say they hadn't been able to locate him.
They found him inside a week! He was living only about 3 miles away, under another name (because of debts attached to the freehold) in a house he'd bought very soon after selling the flat to the owner we'd bought from.
I asked the PIs how they could be sure it was him.
'We have our methods!' Very likely not strictly legal!
But it was him. At first, whoever answered the door at the address given, denied all knowledge of Mr Freeholder, despite his name being there in black and white on the Land Registry.
However, he soon 'appeared', once it dawned on him that there might be money in it.