Could the tap water have enough Fe2+ to precipitate the gold? My well water is packed with Fe2+ right out of the ground (it oxidizes to annoying rusty Fe3+ after sitting in air for a day), and iron pipes are common in city water systems.
If you had a very dilute gold solution, and added ferrous sulfate (the active ingredient being the Fe2+), you'd expect to have a clear solution that tests negative with stannous chloride, because the gold is no longer dissolved ions, but sub-micron dots of inert metal swirling around.
Dissolved ions are the big reason distilled water, or at least deionized or clean dehumidifer water, is preferred to tap or well water in refining. Depending on the process, chloride ions can drag down silver, ferrous ions could reduce and precipitate gold, calcium ions can precipitate sulfate giving a hard to filter and valueless precipitate, etc.