Factual error: There are two rentals cars driven by "Theresa" which have the same North Carolina license tag number, which is displayed on the front where required in Maine, the actual location of filming, but in North Carolina, license plates are not displayed on the front of the cars.
Factual error: Throughout the movie we see several shots of the supposed NC coastline. None of NC's coastline contains giant pines on the beach or rocky cliffs at the water's edge nor does the coastline rise to over 12 ft above sea level for almost 30 miles inland (past the Outer Banks and the mainland sound shore).
Factual error: When Theresa is driving to meet Garret for the first time, there is an aerial shot of a suspension bridge. There are no vehicular suspension bridges in North Carolina. That particular bridge is located in Maine.
Answer: None of the bottles ended up in Lake Michigan. Theresa is from Chicago, and when she and her son are at Boston Logan Airport she tells her ex-husband she'll be staying in Cape Cod for a few days, which is where she finds the bottle on the beach, along the Atlantic Coast. Then after the newspaper article, Theresa learns of another letter in a bottle, which Theresa notes as "bottle #2", and this bottle was found in Ocean City, Maryland, also along the Atlantic Coast. Catherine's letter in a bottle was found at Virginia's shoreline, so all of the bottles were found on the shores of the Atlantic Ocean.
Super Grover