Confirming the early suspicions on the identity of the dark-mantled gull which appeared several times in Costa del Este last weeks, a group of experts (including, among others, Dr. Bob himself) determined it as a sub-adult Kelp Gull. A review of the description plus the photos obtained by Euclides Campos and Ramiro Duque showing the bird with a typical adult Kelp Gull (you can see the photos in Xenornis) helped with the i.d. The diagnostic features that exclude a Belcher's Gull are size (and jizz), pale yellow legs (not bright yellow), dark streaking along sides and flanks, bill shape/pattern, and a mostly dark tail (not a definitive "band"). My error! considering that I have seen both species in central coastal Perú few years ago... my own photos of that trip demonstrate the slimmer shape and its relative small size (specially if you compare it with the nearby Inca Terns in the second photo):
An extensive Google search produced few photos of confirmed sub-adult (third-year) Kelp Gulls, the most useful being those at Martin Reid's website. If you check this group of photos you can see the same bill shape and pattern, the mostly black tail and black primaries with the wings closed, while this other one also shows the mostly black tail. Conclusion: it is a sub-adult Kelp Gull... follow your first impressions.
P.D.: in spite of the several other visits, I failed to re-locate the Kelp Gull(s) at Costa del Este, but I find more Herring Gulls (up to three at the same time) and the Lesser Black-backed Gull that was reported initially by the Ahrens (thanks to Darien and the Kaufmanns). I got only a distant photo.
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