There are a number of things that can cause a stored proc's plan to be dropped from cache. Stats updates, index rebuilds, table alterations, etc. Be very careful relying on what's in sys.dm_exec_cached_plans
http://sqlinthewild.co.za/index.php/2009/07/10/why-the-dmvs-are-not-a-replacement-for-sql-trace/
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability