Magical Focus (Arcane Connections): Minor or Major?

What I meant about "detecting ACs" was a vague memory of reading a spell or guideline that allowed one to learn the existence and/or location of any active arcane connection to oneself (or to some other target) I admit not knowing where I got that impression from.
If this is relevant, then it is an InVi application in addition to the CreVi and PeVi already mentioned.

People keep suggesting it, but to my knowledge such a guideline has never been published.
PArtially becuse it is so problematic - most things are ACs to something, really.

Yes, but most things are not ACs to a spell's specific target.
Outside of your hometown, ACs shouldn't be lying around elsewhere than your current home or places you recently visited. Elsewhere, there might be something fishy, someone actually took it. And if you can target an AC to sever its connection, you should be able to detect it.