I just logged into my NNS and was surprised, that I have a new locked neuron in my NNS, which wasn‘t there yesterday. It has 0 ICP in it and is locked for 7 years and 356 days, with only 0.0013 Maturity staked… What is that neuron, and why is it in my NNS?
There was a bug in NNS dapp where it hides neurons which only have staked maturity.
This bug is now fixed.
This is listed as " * Don’t hide neurons which have only staked maturity." in the change log in this upgrade proposal.
You must have once lost this neuron because you had normal maturity which you then staked.
Now you have it back. 
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