Not quite so undetectable... in theory.

PBS Spacetime - How Many Black Holes Are In The Solar System? https://youtu.be/wh75ubECL8I

And then How to spot tiny black holes that might pass through the solar system - https://www.sciencenews.org/article/primordial-black-holes-s...

That references two papers:

Probing primordial black holes and dark matter clumps in the Solar System with gravimeter and Global Navigation Satellite Systems networks - https://journals.aps.org/prd/abstract/10.1103/PhysRevD.110.0...

Close encounters of the primordial kind: A new observable for primordial black holes as dark matter - https://journals.aps.org/prd/abstract/10.1103/PhysRevD.110.0...