I thought I had gone through the smart Operator's Manual pretty thoroughly when I go my car a few weeks ago but missed this one until going through it again yesterday. Perhaps some others may have also.
Page 84--(Factory default). Vehicle locks automatically when reaching speed of approximately 8 MPH.
If you don't want it to lock automatically:
Swiltch off ignition
Press central UNlocking switch and UNlock button on key simultaneously
Signal sounds, auto lock is switched off.