The barely-there Pro Mesh 2.0 Sleeveless Base Layer is the warm weather version of Castelli's Tour de France winning model, made from light 3D mesh fabric for optimised moisture management in mild or hot conditions.
Unlike many base layers, it has a comfortable flat hem at the waist which won't show under shorts, and it's made to fit under a sleeveless or short sleeve jersey unobtrusively. When you remove that jersey, though, its anonymity will cease, since it's printed with an all-over graphic in fun colours.