diff --git a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollower.h b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollower.h
index 6630fa7250a4ce0ea89c69362edaf6001dfa7fd2..6ef3b1e3a5f2c9c6d22629ebbe6d34287b1b18e0 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollower.h
+++ b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollower.h
@@ -37,6 +37,7 @@ namespace G4UA
 
     struct Config
     {
+      /// FIXME: Public tool handle. See ATLASSIM-3561.
       ToolHandle<Trk::IGeantFollowerHelper> helper =
         ToolHandle<Trk::IGeantFollowerHelper>("Trk::GeantFollowerHelper/GeantFollowerHelper");
     };
@@ -51,6 +52,7 @@ namespace G4UA
 
     Config m_config;
 
+    /// FIXME: mutable helper! See ATLASSIM-3561.
     mutable const Trk::IGeantFollowerHelper* m_helperPointer;
 
   }; // class GeantFollower
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerHelper.h b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerHelper.h
index b5577267e255de7154b5d6d66f3dcba3edfe2b09..2850eb2dd85ebce93b4db341a3598a2fd80189d9 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerHelper.h
+++ b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerHelper.h
@@ -14,6 +14,7 @@
 #include "TrkG4UserActions/IGeantFollowerHelper.h"
 #include "G4ThreeVector.hh" //typedef, can't fwd declare
 
+// FIXME: header-global macro for an array size constant!
 #ifndef MAXPROBES
 #define MAXPROBES 50000
 #endif
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMS.h b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMS.h
index 63276d20d20a98b15798867e6bc06cb574d0ba94..725e87a475f16baaa8f8856928b4d24770ec8193 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMS.h
+++ b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMS.h
@@ -37,6 +37,8 @@ namespace G4UA
 
     struct Config
     {
+      // FIXME: public tool handle in thread-local action!
+      // See ATLASSIM-3561.
       ToolHandle<Trk::IGeantFollowerMSHelper> helper =
         ToolHandle<Trk::IGeantFollowerMSHelper>("Trk::GeantFollowerMSHelper/GeantFollowerMSHelper");
       ServiceHandle<Trk::ITrackingGeometrySvc> trackingGeometrySvc =
@@ -52,7 +54,10 @@ namespace G4UA
   private:
 
     Config m_config;
-    /** tracking geometry */
+
+    /** tracking geometry
+        FIXME: mutables in thread-local action! ATLASSIM-3561.
+    */
     mutable const Trk::TrackingGeometry*         m_trackingGeometry;
     mutable const Trk::IGeantFollowerMSHelper*   m_helperPointer;
 
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMSHelper.h b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMSHelper.h
index 5f18d7825405fab16b9ccb6c666e5271c507bc3a..2fa6f46aad0174c1929acff9356bde3f9625415a 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMSHelper.h
+++ b/Tracking/TrkG4Components/TrkG4UserActions/TrkG4UserActions/GeantFollowerMSHelper.h
@@ -16,6 +16,7 @@
 #include "TrkG4UserActions/IGeantFollowerMSHelper.h"
 #include "G4ThreeVector.hh"
 
+// FIXME: header-global macro for an array size constant!
 #ifndef MAXPROBES
 #define MAXPROBES 50000
 #endif
@@ -52,6 +53,7 @@ namespace Trk
       void trackParticle(const G4ThreeVector& pos, const G4ThreeVector& mom, int pdg, double charge, float t, float X0) const;
 
       /// modify TSOS vector
+      /// FIXME: const by-value vector argument!
       const std::vector<const Trk::TrackStateOnSurface*>
       modifyTSOSvector(const std::vector<const Trk::TrackStateOnSurface*> matvec,
                        double scaleX0, double scaleEloss, bool reposition,
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorder.cxx b/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorder.cxx
index 5bee85fcb580a15a8c36be6e0e9f1841578ee923..dcdee9a9f0ed84c310bce251dd659efbd0d66ebc 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorder.cxx
+++ b/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorder.cxx
@@ -28,6 +28,7 @@ namespace G4UA
 
   }
 
+  // FIXME: Why implement run action if empty?
   void EnergyLossRecorder::BeginOfRunAction(const G4Run*)
   {
     return;
@@ -46,6 +47,8 @@ namespace G4UA
   void EnergyLossRecorder::EndOfEventAction(const G4Event*)
   {
     if (m_config.pmWriter) {
+      // FIXME: thread-unsafe usage of a component in a thread-local action?
+      // See ATLASSIM-3562.
       m_config.pmWriter->finalizeTrack();
     }
     m_entries = 0;
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorderTool.cxx b/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorderTool.cxx
index be2dd522fc41eb9bd68ae4882b497e12eb939864..9864f3f7e15a8deced3c2123a3cfdbace138047f 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorderTool.cxx
+++ b/Tracking/TrkG4Components/TrkG4UserActions/src/EnergyLossRecorderTool.cxx
@@ -23,6 +23,8 @@ namespace G4UA
   {
     if(!m_pmWriter.empty()) {
       ATH_CHECK(m_pmWriter.retrieve());
+      // FIXME: thread-unsafe usage of a component in a thread-local action?
+      // See ATLASSIM-3562.
       m_config.pmWriter = &(*m_pmWriter);
     }
     return StatusCode::SUCCESS;
diff --git a/Tracking/TrkG4Components/TrkG4UserActions/src/GeantFollowerMS.cxx b/Tracking/TrkG4Components/TrkG4UserActions/src/GeantFollowerMS.cxx
index cdb883d857c63bec7ffb136baf73d9e8a4634b36..816d3a91572dc37ccb93bed68a4e8f6912344681 100644
--- a/Tracking/TrkG4Components/TrkG4UserActions/src/GeantFollowerMS.cxx
+++ b/Tracking/TrkG4Components/TrkG4UserActions/src/GeantFollowerMS.cxx
@@ -31,11 +31,13 @@ namespace G4UA
 
   void GeantFollowerMS::BeginOfEventAction(const G4Event*)
   {
+    // FIXME: thread-unsafe tool usage in thread-local action? ATLASSIM-3562.
     m_helperPointer->beginEvent();
   }
 
   void GeantFollowerMS::EndOfEventAction(const G4Event*)
   {
+    // FIXME: thread-unsafe tool usage in thread-local action? ATLASSIM-3562.
     m_helperPointer->endEvent();
   }