It is unclear who was at fault and whether it is Sanchez or Arsenal, who could be facing a possible sanction.
There is even confusion about whether doping control officers were at Arsenal’s London Colney base.
Initial reports in Spain said testers from UEFA had arrived and requested a sample from Sanchez only to discover he was not there, but UEFA on Thursday night denied any involvement.
More likely is the possibility that Sanchez has fallen foul of ‘whereabouts’ rules, having travelled to complete his move to United, when his whereabouts details still showed him in London.
The FA and Arsenal declined to comment on Thursday night.
An individual can face up to a two-year ban if he has three whereabouts failures in 12 months, but it is not known if it is a first, second or third strike for either Sanchez or Arsenal.