I had detached a SolarWinds database called NetPerfMon that consisted of three data (mdf) files and one log(ldf) file.
That might be the problem. Detaching a db in SQL 2005 and later versions changes the permissions on the file. So it might be a permissions issue trying to read the existing files.
For that reason and others, I suggest using "SET OFFLINE" instead of detaching whenever you can.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.