Zipcar operate a "flex" system in London, which means you can drop the car off, lock it via an app, and leave it for the next person to pick up. So I didn't see the PCNs as I wasn't the next person to use the car.
This was Zipcar's response to my email asking for clarification:
"Thank you for contacting us.
Kindly be advised photographic evidence of the contravention can be viewed on the issuing authority's website by entering the violation reference number and the vehicle registration number:
https://haringey.tarantoportal.com/PCNs/.
In addition please be kindly advised that when a penalty is first issued and placed on the car there is usually a 14-day window within which a reduced rate of 50% is payable. If we are made aware of any notices found in our cars we make all efforts to process them within that period, so we can take advantage of the reduced rate.
However, we only received the Notice To Owner for this violation, so it was settled and applied to your account at the rate it was received. And as the liable driver you are responsible to cover the cost set by the issuing authority.
We understand that this is a frustrating situation, but we need you to remember that PCNs are not issued by Zipcar, but by the local authority. We try to accommodate our members as best we can, but we, as a company need to recover the costs of these fines from the members responsible.
We hope this has been helpful."