He had not been told that Dotcom's lawyers had questioned it in court a month earlier and there had been media reports alluding to it.
He had also not been told that while he was overseas his deputy, Bill English, had signed a document for the court testifying to the agency's involvement in the case in a bid to have its involvement suppressed.
It is surprising English did not twig at that point. If Dotcom's residency status was vague at the time of his arrest in January, his high profile since meant it was well known.
The first Key knew anything was last Monday - five days after the bureau apparently first realised it had exceeded its powers.
He professed himself "quite shocked". But he also tried to assert that such ignorance on his part was nothing out of the ordinary. Even more disturbingly, he said the fact that he had not known about it meant he had no responsibility for it.
Key's aim is seemingly to achieve a state of deniability: what he does not know about, he cannot be held accountable for. Such blissful ignorance is politically convenient, but there is a point at which it goes too far.
Key is effectively asking the public to trust him about the GCSB after admitting he had no idea what they were doing and was effectively assuming they could be trusted himself.
The astonishing revelations and Key's belief that it is normal that the Prime Minister should be so blind to the GCSB's actions makes it hard to accept his assurances that the bureau is otherwise exemplary.
It is not known what information the GCSB obtained, who from, what it was used for. The memorandum filed in court shed some light, stating only communications that related to the whereabouts of those in the police sights at the planned time of arrest were actually sent to police.
However, it also made it clear that the GCSB had gathered a wider range of material and was silent on whether that could be used in the case against Dotcom.
The Prime Minister has so far refused to answer such questions, saying he prefers to wait until after a report by the Inspector-General of Intelligence and Security is delivered at the end of the week.
Key is not the only line between that bureau and abuse of powers - the inspector-general is specifically charged and empowered to ensure both the GCSB and the SIS do not exceed their limits.
But Key is the minister responsible for the agency for a reason. He cannot shelve away his role as one of the relatively few checks and balances on intelligence agencies which are justifiably exempt from the usual public scrutiny - but do nonetheless need some monitoring.
When the report by Paul Neazor is delivered - and Key has clearly already been briefed on its contents - Key will have to decide how much of it can be released. Quelling suspicion about the agency and concern about his own oversight would encourage a detailed release of Neazor's findings, rather than a heavily redacted version.
The litany of cockups by New Zealand authorities in the Dotcom case is so long now that it is tempting to prefer the conspiracy theory over the cockup theory, simply because that at least does not make us look quite so stupid.
Meanwhile, the Opposition can simply sit back and laugh. For in his brief time here, Dotcom has almost single-handedly done what Labour and the Greens have failed to do for four years - embarrass the Government and dent Key's credibility.
The Opposition might consider Dotcom the gift that just keeps on giving. But Key must be wishing he could rewrite the laws to give the GCSB the same statutory powers to deal with Dotcom as the Oompa Loompas had to deal with Gloop: after he was sucked up a pipe, they chopped him up, boiled him and made him into fudge.
