Went into the Simplify environment, clicked on the Model and the assembly disappeared.
Solution
Right click on the <assembly name>_Simplify entry in PathFinder and select Hide Previous Level. This button is a toggle and should display the assembly.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 1064
Product: SOLID_EDGE
Application: ASSEMBLY
Version: V219MP9
Function: SIMPLIFY
Ref: 001-9581499