Main-sequence dippers in Gaia epoch photometry: a quality-flag filter, a DR3 null result, and pre-registered predictions for Gaia DR4

We search Gaia DR3 epoch photometry for main-sequence stars with rare, deep, aperiodic dimming events (analogues of Boyajian's star). In 6863 main-sequence stars (G < 16) with G-band skewness > 2 and no explained variability class, most apparent dips are caused by single transits whose g_other_flags field has most of its ten lowest bits set; these are not removed by the official rejection flags. Removing the transits with all ten bits set eliminates 74% of the dip events, and a generalised rule (five or more of the ten bits) cuts the strong candidates of a 187-star sub-sample from 139 to 27. After multi-transit colour-coherence, scan-angle, triage and catalogue checks (VSX, SIMBAD, ASAS-SN, ZTF, AllWISE, YSO catalogues), the 27 survivors are 15 young stars, 3 eclipsing binaries, 3 RS CVn binaries, 2 spotted rotators and 4 unexplained objects. The best of them, Gaia DR3 5219597583589736832, shows a ~5% dimming lasting ~100 d in late 2015 without infrared excess; it does not recur in ASAS-SN (2016-2018) or in nine TESS sectors (2019-2025). No old main-sequence dipper is confirmed in DR3. Before the Gaia DR4 release we fix the full pipeline, a 145-object control sample (the 27 DR3 survivors, the 81 ZTF main-sequence dippers of Tzanidakis et al. 2025, Boyajian's star and the 36 candidates of Schmidt 2019 and 2022), five falsifiable predictions, three reported controls and the script that tests them (c_verifica_dr4.py). The dominant artefact is probably the bright-star 'big dipper' effect known from Gaia Science Alerts (Hodgkin et al. 2021); we identify its signature in the published epoch photometry. The analysis was carried out with the assistance of an AI model (Claude, Anthropic); see the section "Author contributions and use of AI" in the note.