Problem is, a fetish can be *anything*. Try noticing the acorn clenched in the fist of a druidic shaman while he's got a gun in the other hand.
The way I rule it is that the mage has to play with the fetish like a catholic or Buddhist might twiddle with a rosary or a prayer bead respectively, so the hidden under armour thing won't work at my table (you would have to pull that necklace from out under your armour and twist it as you incant your spell). The exact trappings might vary from tradition to tradition, but I treat all fetishes as obvious magical props, although like Yera I do not consider them "active" like foci. I am aware these are house rules specific to my game, and yourself, Halinn and Neraph are all perfectly correct by RAW.
Fetishes also make smuggling compartments for cyberlimbs or the skin pouch Bioware attractive.
They most certainly do!