From their report:
The Mirror used drug testing wipes on a surface behind a toilet cistern in a public area of the Cambridge Wing, yards from the Lindo Wing maternity ward at the hospital.
It showed positive for the Class A drug.
The screening equipment, used by police and the NHS, turns blue to indicate traces of powder or crack cocaine crystals.
The Mirror passed on the information to royal officials and health bosses.
Kensington Palace said tonight that it was “a matter for the hospital”.
Kate, 33, and Prince William, 32, had their first child, George, at St Mary’s in July 2013. It has been the birthplace of generations of the Royal Family and the facilities are thought to be among the best in the world.
Meanwhile, spokeswoman for Imperial College Healthcare NHS Trust said: "We take the safety of our patients and staff extremely seriously and we will work closely with the police to address the issue."
Really strange and suspicious. I wonder why anyone was testing for drugs in the first instance.