Right, I should have narrowed it down more: we can query series which a) have been curated before (have a disk watermark), or b) have received a memory sample already during the current Prometheus run. The edge case are timeseries which have not been curated and which haven't been seen yet since the last restart. This can happen if you briefly run a new Prometheus, then restart it. Suddenly you are not able to query the samples from the first run anymore (until you receive more of the same timeseries).