:help syntax-loading has detailed description of how syntax gets set up. For instance this section on file loading:
Loading the file triggers the BufReadPost autocommands.
+- If there is a match with one of the autocommands from |synload-3|
| (known file types) or |synload-4| (user's file types), the 'filetype'
| option is set to the file type.
+- The autocommand at |synload-5| is triggered. If the file type was not
| found yet, then scripts.vim is searched for in 'runtimepath'. This
| should always load $VIMRUNTIME/scripts.vim, which does the following.
| +- Source the user's optional file, from the *myscriptsfile*
| | variable. This is for backwards compatibility with Vim 5.x only.
| +- If the file type is still unknown, check the contents of the file,
| again with checks like "getline(1) =~ pattern" as to whether the
| file type can be recognized, and set 'filetype'.
+- When the file type was determined and 'filetype' was set, this
| triggers the FileType autocommand |synload-6| above. It sets
| 'syntax' to the determined file type.
+- When the 'syntax' option was set above, this triggers an autocommand
| from |synload-1| (and |synload-2|). This find the main syntax file in
| 'runtimepath', with this command:
| runtime! syntax/<name>.vim
+- Any other user installed FileType or Syntax autocommands are
triggered. This can be used to change the highlighting for a specific
Perhaps you can piece things together by going through this.
Edit: Moving the winning idea from a comment to here...
As for a more concrete idea, if you do
:hi you can try to match the text colors with a syntax group and see if that gives you some clues. First thing that comes to mind as far as coloring of plain text is search or incremental search. The next one that comes to mind is spelling errors or warnings. (If it's spelling that can be checked by simply doing typing ]s and seeing if the cursor moves to a highlighted word.)