While I value wikipedia as a source of information you must realize that it is edited and published by individuals like you and I. All information in Wikipedia is not accurate.

There are instances where the Alligator Snapper was found in Michigan and it was noted as a Nonindigenous occurances.

Interactive maps: Continental US, Alaska, Hawaii, Caribbean

Nonindigenous Occurrences: M. temminckii was captured from the Reflecting Pool, in the Mall area of Washington, D.C., in August 1994 (Katz, 1994).

In the St. Joseph River, Berrien County, Michigan, M. Brazo (personal communication 1999) found an alligator snapping turtle in 1999.

In Phoenix, Maricopa County, Arizona, a M. temminckii was observed at Papago Park Ponds in 1987 and 1991 (M. Demlong, personal communication 1997).

A M. temminckii was collected near Elk Grove, Sacramento County, California (Bury and Luckenbach, 1976).

Means of Introduction: Unknown; but in all cases pet releases should be suspected.

Status: Inexplicably, the alligator snapping turtle found in Washington, D.C., was scheduled to be "relocated" to Roosevelt Island or in waters along the George Washington Parkway, D.C., according to Park Ranger E. Broadbent (in Katz, 1994); not established, but still well away from its indigenous range.

Macrochelys temminckii collected from Michigan and California, and observed in Arizona are probably individual releases or escapees, and not known to represent breeding populations.
If you indeed have a Alligator snapper it is really rare. It needs to be reported.