Rollers on the 400 have a poorly designed proximity switch which has a long history, the fix by some airlines has been to check the rigging more frequently to prevent unwanted reverser messages, air load and/or thrust loads can cause a slight shift of the reverser as you could expect as components wear which sets off the proximity switch. Locking out the reverser doesn't fix the problem always either as the wear and or rigging won't be fixed by a locked out reverser, so you lock it because of the faulty message taxi out set thrust get the same message. Delay flight to re rig reverser? You can't change the reverser either as it requires the entire engine to be changed.
Experienced RR guys have probably seen all this before so know or try to do what they can to prevent unwanted reverser messages.
Was any work carried out on any of the reversers while in JNB?
The 'air gap' will also be a feature of the new 747-8 I/B Kreuger flap it's being redesigned for this purpose.