Just saying.
<https://abcnews.go.com/blogs/headlines/2014/05/ex-nsa-chief-...>
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.
<https://www.schneier.com/blog/archives/2014/03/metadata_surv...>
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.
<https://en.wikipedia.org/wiki/Mail_cover>
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..
<https://en.wikipedia.org/wiki/Mail_Isolation_Control_and_Tra...>
39 CFR § 233.3 - Mail covers. <https://www.law.cornell.edu/cfr/text/39/233.3>
(I was ... vaguely aware of this.)