Mantis#2796. Thank you kindly, Gerhard for a patch that addresses:

On a call of llVolumeDetect(1) (or any other number !=0) volume 
detection is enabled. Together with VD, the phantom flag is set to the GUI.
On a call of llVolumeDetect(0), vd detection is switched of again, 
also the phantom state is removed. On a call to llSetState(STATE_PHANTOM, 
false) while VD is active, also VD is switched off. The same is true for 
unchecking the phantom flag via GUI. This allows to take back VD without 
the need to script just by removing the phantom flag.
Things missing in this patch: persistance of the volume-detection flag. 
This needs more discussion and will be included in another patch soon.

OpenSim/Region/Physics/OdePlugin/ODEPrim.cs

@@ -127,6 +127,8 @@ namespace OpenSim.Region.Physics.OdePlugin
         private bool m_isphysical = false;
         private bool m_isSelected = false;
 
+        internal bool m_isVolumeDetect = false; // If true, this prim only detects collisions but doesn't collide actively
+
         private bool m_throttleUpdates = false;
         private int throttleCounter = 0;
         public int m_interpenetrationcount = 0;
@@ -2226,7 +2228,10 @@ namespace OpenSim.Region.Physics.OdePlugin
 
         public override void SetVolumeDetect(int param)
         {
-
+            lock (_parent_scene.OdeLock)
+            {
+                m_isVolumeDetect = (param!=0);
+            }
         }
 
         public override PhysicsVector CenterOfMass