This is probably a reference to US postal or mail covers.
The USPS takes images of most or all postal mail as part of its delivery and postal sorting/routing processes. Those covers are retained for a limited period of time, and actually have, so far as I understand, significant privacy protections associated with them, of the sort notably absent in most electronic communications.
See:
Mail Cover (Wikipedia):
Mail cover is a law enforcement investigative technique in which the United States Postal Service, acting at the request of a law enforcement agency, records information from the outside of letters and parcels before they are delivered and then sends the information to the agency that requested it.[1] The Postal Service grants mail cover surveillance requests for about 30 days and may extend them for up to 120 days.
MICT: Mail Isolation Control and Tracking (Wikipedia):
[A]n imaging system employed by the United States Postal Service (USPS) that takes photographs of the exterior of every piece of mail that is processed in the United States.[1] The Postmaster General has stated that the system is primarily used for mail sorting,[2] though it also enables the USPS to retroactively track mail correspondence at the request of law enforcement.[2] It was created in the aftermath of the 2001 anthrax attacks that killed five people..
You can sign up to have them email you a daily summary of your mail deliveries including the associated images they've logged under USPS Informed Delivery.
As Bruce Schneier has noted, metadata equals surveillance, as it's actually far more amenable to analysis and inference than whole-text or audio capture. Though that latter may have shifted significantly with the rise of LLM AI techniques.
