You are seeking a lady whom you have found in the 1910 Census, but not in the 1920 Census. Have you checked all the 1920 Census or just the part for Michigan? In the UK, nuns tended to be recorded in the Census under their civil name.
You believe she drove a taxi in Ypsilanti. Taxi drivers are often licensed by the civil authorities. Have you checked the Ypsilanti City rexcords to find when her license lapsed, as this might indicate a year by which she had entered a convent?
Turning to Catholic records, in 1908 there was a papal document, "Ne Temere", that required certain other sacramental details to be recorded in the person's baptism record. As you have found her in the 1910 Census, I would GUESS that she had not joined the Catholic Church at that time, and that she became a Catholic and entered a convent within the next 10 years. When she joined the Catholic Church she was probably conditionally baptised - if she had been validly baptised earlier then this conditional baptism would have been void. This baptism would be recorded in the Baptism Register of the church where it took place, and later details of her religious profession as a nun should have been recorded alongside it.
Rather than trying each parish that was in existence in 1910 - 1920, you could try the Lansing Diocesan Archivist, as Ypsilanti is in the Catholic Diocese of Lansing. The Diocese of Lansing was only established in 1937 and its history is outline on its web-site (http://www.dioceseoflansing.org/about.html
). There is also a page about the Diocesan Archives (http://www.dioceseoflansing.org/archives/index.html
) that explains that sacramental records are held in the original parish, not in the archives, and there is a note about the conditions for access. There is very limited access for records less than 90 years old. Your search would relate to records 90 - 100 years old, so access should not be a problem. You should be asking the Archivist about parishes in existence ca 1910 - 1920, and, as White-Roses suggests, which convents existed at that time. It is possible that there is a diocesan list of ladies entering religious life, but as the diocese was only founded in 1937, it may not go back that far. The Diocese of Lansing was established out of the Archdiocese of Detroit and the Diocese of Grand Rapids, and you could ask the Archivist which you should contact for earlier information.
Jim Lancaster (Bury, Lancashire, England)