What is retroactive surveillance (Rückwirkende Überwachung), what data does it yield, and what is its key limitation?

Question

Accepted Answer

It targets one subscriber (one SIM) and retrieves stored metadata about past communications — call/SMS partners, timestamps, durations, data connections, and the serving antenna's ID, location, and azimuth — but only as far back as the retention period allows (6 months in Switzerland).
The target is a single subscriber, i.e. one SIM. What comes back is the metadata of past activity: incoming and outgoing calls and SMS (each with the partner, the timestamp, and the duration), the data connections, and the ID, location, and azimuth of the antenna the device was on — which together sketch an approximate location trail over time.
The hard limit is the retention period. In Switzerland the data is kept for 6 months; many other countries mandate anywhere from 6 months to 2 years. You can never look back further than the data was actually retained — no retention, no retroactive view — so the window itself bounds the entire technique.
It is essential to see what this is not: retroactive surveillance yields metadata, not content. It tells you who talked to whom, when, how long, and roughly where, but never what was said — listening to the actual content is real-time surveillance, which carries a much higher legal bar. And yet metadata is powerful precisely because a complete map of someone's contacts, timing, and movements over months can be more revealing than any single conversation. That is why retention periods are so politically contested and why the future of data-retention rules (notably in the EU) remains an open policy debate.
Tip: Retroactive = "the past, metadata only, limited by the retention window."
Go deeper:

Data retention (Wikipedia) — what call/traffic metadata gets stored, for how long, and why the retention window is the hard limit on any retroactive view (kept: feeds the carousel).
 Digital Rights Ireland (Wikipedia) — the 2014 CJEU case in which the Court declared the EU Data Retention Directive invalid for disproportionate interference with privacy; the concrete ruling behind "the future of data-retention rules is an open debate."
 How Digital Rights Ireland litigated the Data Retention Directive — and won (EFF) — the eight-year campaign that overturned the 6-to-24-month EU rule, told from the inside; why metadata retention is so contested.