It's always possible that this table is populated by something external to the database instance that the table is situated on - from an application, via a linked server, from a script on the server. There are probably other possibilities but you get the idea.
The best way I can think to track activity on the table is to run SQL Server Profiler (via the Tools menu of SSMS on the 2012 version). I'm not going to write a tutorial on that software here but you essentially want to filter the activity on the server by the name of the table that you are interested in. If anything updates that table then it should appear in the profiler results.