I got Fats Domino's FBI file, and it consisted of a single page, which was a document titled "Subject: Martin Luther King" and just listed New Orleans reports on Domino's three Cadillacs https://foiathedead.org/posts/fats-domino/

I looked up the preceding document in MLK's FBI file—the 17k page one, not the 200 page one the FBI positions as the full thing in its "FOIA vault"—and found that agents had staked out a hotel in Philadelphia looking for MLK, and saw Mr. Domino's car and mistakenly drew a connection.

The hotel the two men each stayed at, the Franklin Motor Inn, was knocked down some time in the 2010s and was replaced with a Whole Foods, but used to be quite a place. Philadelphia Mag did a nice story about its last days as a Best Western bar https://www.phillymag.com/news/2013/02/06/philadelphia-best-western-bar-bw-whole-foods/