Aren't there metals that are undetectable/less detectable, like titanium or even stainless steel?

No, it doesn’t work like that in the modern world. The nature of the materials are pretty obvious to a remotely competent sensor.

using what physical process? How exactly do they detect any metal? A genuine question.

	▲	kube-system 2 days ago \| parent [-]
		You can detect nonferrous metals by inducing eddy currents in them, then they become magnetic.