So there's a lot to unpack here but I'll give it a try:

1. You could have native ancestry even if it doesn't show up in a DNA test. Consider a person with ancestry A has a kid with someone of another ancestry, whose descendants do not reproduce with another person with ancestry A. Then that ancestry would logically show up in these percentages: 100 50 25 12.5 6.25 3.125 <2 <1... But this assumes that it divides evenly, which it does not. Even if it did though, in this example that ancestry might be undetectable after about 8 generations.
2. There's a lot of "I have a native ancestor" narratives out there. Why? Claims to American legitimacy, alleviation of colonial guilt, that one guy in a feathered headdress cried about littering and I'm sad about it too, etc.
3. It doesn't matter. Ancestry is pointless. Your DNA is just a listing of the traits your body was originally constructed on. If a native couple adopt a white kid and raise them on a reservation that child has had more of the native experience than a white passing person of native ancestry raised in Boston. Even more importantly though, none of it changes the fact that colonialism was a crime, we all should have empathy for its victims, and the way forward is by treating people with respect and dignity and trying to repair the damage while preventing it from happening again.