The key to the clues (spoilers for episode 1)

Having recorded this last year, I have finally caught up with it.

I'm enjoying it, but one thing really irritated me.

The breakthrough that leads them to all subsequent clues is the car key they find under the body. This is then traced to the car, which gives the address book and everything else. Without that key, the police had nothing.

But then we learn that the car was "stolen from outside the house". So how come they had the original key, that matched the car sale records?

I realise that the key may have been left in the car, allowing it to be stolen. But I would have expected this to have been asked, at least, during the investigation - it is a fairly important issue, as it raises the possibility that the car had not been stolen at all, but given to someone.

Am now on episode 4, and had been hoping that this would be resolved or mentioned by now, but no. The key has served its purpose and is now largely forgotten.