Whoever authorised these attempted murders were worse than amateurish by leaving such an attributable "calling card" at the scene of the crime.
It appears the toxin was administered in Russia to a possession belonging to Yuri who had just returned to the UK from a visit there.
I've never heard of detectives being first responders to such a scenario. Unusual to say the least. Adds to the conspiracy theories I suppose.
Detective Sergeant Nick Bailey was not first on the scene at the park bench where Sergei and Yulia were discovered. He was assigned to the investigation and attended their home on the outskirts of Salisbury.
Conspiracy theories tend to vanish in the light of facts.