Add timestamps to user documents
Most database technologies store this metadata by default.
Because the expected data volume and change rate of this attribute will most probably be low, there should be no reason of not storing this information.
Of course this information might already be available in audit files, but first: auditing isn't enabled by default.
Second: most database users won't have access to this file/info and third: most users won't expect this info in a separate file (reminder, MongoDB recommends to store the data where it belongs when it comes to "data/schema modelling", so the metadata of a user document should also be located within this document!).