Suspected poisoners Alexander Petrov and Ruslan Boshirov went to the Czech Republic when Skripal could have been there. The British say they were tracking him – but maybe they were tracking the Novichok nerve agent, which Porton Down and the Czechs had.
In the Netherlands, the supposed GRU agents arrested outside the OPCW HQ were actually IT experts. They were checking the security of the Russian embassy’s communications, which had been bugged. That they were by the OPCW building is coincidence – the Russian embassy is just across the road.
The Dutch authorities presented numerous evidence about the real identity of the GRU agents and the real purpose of their trip - like taxi receipts for the trip from the GRU building in Moscow to the airport, passport scans, and the specialist equipment which the GRU agents tried to destroy when they were caught. See e.g. https://www.theguardian.com/world/2018/oct/04/netherlands-halted-russian-cyber-attack-on-chemical-weapons-body