I have an idea that I thought I’d bounce off the board here.

Many foci are rather easily recognizable as such. Whether it’s your “Gandalf Staff” with a crystal on top that just screams “this is a magical staff” or some sort of clearly Native American ceremonial fetish, it’s nature is fairly clear by its design when you’re walking around with it, and that’s not always a good thing; except for those times when it is. Whether for purposes of personal style or for intimidation, there are occasions that letting someone know that you’re pointing an obvious magical weapon at them is useful. However, walking through airport security is not one of those occasions. Can you have your cake and eat it, too?

A useful tool might be something that isn’t recognizable as a focus – until you want it to be. A new advanced focus – the Shapechanging Focus.

It has two forms. In its true, mundane form, it’s presumably something innocuous that you could carry around with you anywhere and not raise any suspicions. But when the focus is activated, it changes form into something else.

To prevent ridiculous abuse, this should be something of at least roughly the same weight, and shouldn’t be anything of an advanced technological nature. A full-sized example of the item the focus turns into when activated should be a necessary component of the enchanting process. I’m not sure what the KP cost should be.

Your thoughts?

It's a good thought, and there is a sort of appeal to having an item change when it 'powers up', just not sure SR magic at it's current stage is up for it.

The mass shift limitation is probably a good idea, though I find the idea of a set of 3 rings that transform into a gauntlet, a sword and a wand respectively still appealing.

I would probably still allow someone assensing the user with the in mundane mode to be able to detect it with 5+ hits on the assensing test , similar to when looking for deltaware and similar obscure details, so it is a lot less noticeable in mundane mode, but not a total free ride through security.

A good comparison. It's not immediately noticeable, but if they really go looking for it...

Still, it's better than, say, openly carrying around a big glowing sword or something, which announces to the world that you've got a magical item all the time it's not hidden away from view.

Well, I never understood why a weapon focus has to glow puple and look like an ancient sumerian sword. Why not just enchant a trusty combat knife, or a store-bought pair of hardliner gloves? Same with other foci. I just don't see any reason for making them ancient greek style pendants the shape of a naked elf that glows in the night - why not enchant your favourite bottlecap instead, or your trusty fedora, or your favourite pair of shitkicker combat boots. The way I see it, the magically magical-looking artifacts of magic are a genre convention neither covered by rules nor by ingame nescessities.

Just like wearing dayglo hermetic patterns on your clothes, this is you asking to be geeked first. And there is just no reason to.

True enough, but I still think the ability to turn harmless Item A into less-harmless Item B (only when you want it to be Item B) has to have some uses.

I suppose it does but does it have enough uses to justify the effort to come up with rules for it? I don't know. Personally I find most magicians, either from pre-made adventures or ones I've made up, opt to keep their magic items inconspicuous when they can. Hermit has good points there. No need to advertise that you are rocking the foci and invite a hail of gunfire.
I imagine the magicians that go for the more ostentatious displays of power also tend to be the ones not getting shot at routinely and likely have licenses for all their gear anyway.

Even a fishing rod is pretty effective:

You mean just like how an ally spirit can function as a power focus, and disappears from the physical realm whenever convenient? Introduce unpleasant consequences of dikoting the ally or having sex with it.

True, but foci don't talk back, and this would almost certainly be cheaper than an ally spirit. In general, I trust foci more than spirits, but I'll admit that's my Hermetic bias.

You know, I just can't get this out of my head. I think of the anime Soul Eater, and I consider the possibility of an ally spirit being able to take the form of my favorite focus. Between your idea and mine, might be the ideal solution.

Sure, a decent ally will be more expensive even when you combine it with the Familiar ordeal for initiation. But it might work out being cheaper in the long run if foci get confiscated / poached a couple of times. Of course, you could always say your focus is a glass bead kept inside a 0.0 essence Tooth Compartment and hope you never get cavity searched...

Or an enchanted, implanted, fully internal piece of cyberware, if you're a cybermage. Should be really hard to find, too.

Talk about putting your mana where your mouth is.

Sounds like you are describing a Mask spell. Combine the focus with an Anchoring or Sustaining focus holding the Mask spell.