I disagree. I went into Hero of Ages and did some research. Vin could definitely sense when Ruin manifest via bronze.
That's not entirely surprising when you think about it. Allomancy sends ripples through (presumably) the Spiritual Realm. The specifics aren't important, but Allomancy just focuses a divine power in a very defined way.
Ruin would be extremely obvious to any Seeker nearby. It is just sending a crapton of pulses out by simply existing. It's that divine form, similar to Allomancy, only vaster. But, at the same time, his "bronzepulses" aren't specific. Vin was sort of confused by this when Ruin was acting as a Mistborn. It's not too surprising, considering a Shard could influence a lot more than Allomancy could.
So, I like this theory. I worry about the definition of Investiture, because does this mean a Seeker detects the metalminds by themselves, or Hemalurgic spikes, or just the act of a Feruchemist storing or tapping?
Huh. Now that I think about it, maybe the difficulty of detecting Feruchemy is that the Investiture aren't obvious, like Allomancy. Its the metals which are Invested. So you'd need to try and detect some weird frequency to see the pulses of the Invested metalminds.
Anyways, besides that, I like the general idea.