Full text index search returns an unexpected result
This is a known issue when using SQL Server 2008 to search for a number with more than nine digits. When using certain word-breakers, the number is stored in its scientific form e.g. nn5d68952e+009, leading to false-positive matches. One of the word-breakers affected is the neutral language (lcid 0):

-- Result includes nn5d68952e+009
SELECT display_term 
FROM sys.dm_fts_parser('"5689523657*"', 0, 0, 0);


A language that does not behave this way is us-english (lcid 1033):

-- Result includes nn5689523657
SELECT * 
FROM sys.dm_fts_parser('"5689523657*"', 1033, 0, 0);


There are several other workarounds and fixes mentioned in the KB article link above.