Asymptomatic cases - how far back to trace?

[NeedlecordNoddle]

Considering the news of the school in Staffordshire where 40 kids tested positive but were all asymptomatic. Does anyone know when an asymptomatic case is considered to be "a case" ie. infectious for the purposes of contact tracing?

The UK guidance defines a contact as anyone who has been in contact "2 days before to 10 days after the onset of symptoms" but asymptomatic cases have no onset of symptoms, or may not until later. Do they treat the day of the positive test as the "onset of symptoms"? What if they develop symptoms 3 days later, while they are isolating - does that mean they have no contacts for the purposes of contact tracing?

Thinking this through - say A and B share a taxi Sunday night. A and B have no further contact after that night. They are both tested Weds (in a mass test, like if they work somewhere with regular testing or they are in the school in Staffordshire). Results come back Friday, A is positive and B is negative. Is B a "contact" of A and has to isolate, even if A has never had any symptoms? How far back do they go?

Feels like some kind of GCSE maths question!!