The DexNav Taillow curiosity occurs in Pokémon Omega Ruby and Alpha Sapphire.
In Mauville City, near Square Tower, the DexNav will indicate a nearby Pokémon south of the Poké Mart, which is a Taillow even though a Taillow is not in sight.
It is probably the Taillow on Wattson's balcony directly above that location in Mauville Hills. Additionally, from the Square Tower if the player moves west, and then goes north up the stairs (twice), they can find another Taillow, but that Taillow seems to be directly above the PokéMart and not south of it.