No, I've never seen someone sit in the middle lane when there are no other cars around.
So far you've recommended undertaking on the motorway and told us that you drive with a defective speedo. Please do excuse me if I don't take your advice on safe driving practice too seriously
10% over-read on a car speedo is fairly normal. Some makes are worse for it than others, a Nisaan Terrano I once owned needed to be showing just over 80mph on the speedo to do a real 70mph as measured by GPS.
Charlie, just wondering where you live when you say you've never seen "someone sit in the middle lane when there are no other cars around".
The motorway network in northwestern england (M62, M57, M6, M56, M60) must have your share then!