arXiv:2611.09576v1 Announce Type: new Abstract: Sleep spindles are a hallmark of healthy NREM sleep whose frequency, amplitude, density, duration and topography vary with brain maturation, age and disease. Their manual annotation is impractical and variable between scorers, yet the public references used to train automatic detectors cover these characteristics only partially: stage N2 only, a single central derivation, or short pre-selected segments. Measuring duration and density requires dense detectors that learn event boundaries from annotated onsets and offsets, and therefore inherit the protocol under which those boundaries were scored. We introduce Expert Spindles in the Wild (ESW), a stage-agnostic expert spindle annotation set covering 19 complete MASS-SS2 recordings ($\approx$150 h of continuous EEG, 29,819 events). A sleep neurophysiologist annotated the recordings using the full EEG montage, blinded to sleep-stage labels and prior spindle annotations. On the 15 nights shared with historical scorers, ESW recovers 92.7% of E1 and 73.7% of E2 events, while recovering 81.6% of the MODA expert-consensus events on the common excerpts. Despite stage-blind annotation, 99.0% of ESW events occur in N2/N3. Used to train a fixed dense detector on a frozen SleepFM encoder, ESW reaches an event-level $F_1$ of 0.680 on 166 unseen MODA subjects, compared with 0.635 for E2 ($\Delta F_1$ = +0.045, paired-bootstrap 95% CI [0.024, 0.065]). ESW is thus useful supervision for cross-dataset detection, and closer annotation-level agreement with the MODA consensus did not translate into better transfer performance in this experiment. The annotations are released under the MASS data-use terms (https://doi.org/10.5281/zenodo.23894975).