There is no specific assembly called Microsoft.MSE.Behavior. There are a number of behavior assemblies such as Microsoft.MSE.Behaviors.Security.dll which is available in the security guide. Source code for other behaviors is not available.
I see you posted about the BAM behavior which I suspect is the source code you'd be looking for. We currently don't publish much guidance around the existing BAM behavior.
Feel free to continue posting other questions about the behaviors and we encourage everyone to create their own and post their experiences.