Two thoughts on this. I have come across aftermarket upgrades to alarm systems which cause a car to relock itself if it has been unlocked but then no door is opened. Is your Galaxy old enough (3 years or more, somebody please correct me if the figure is not right) to have the electronic module, controlling the alarm and locking ,housed under the front passenger seat? (In a cavity covered by a hatch between the seat runners) If so the module may have got wet due either to leaks, or due to valeters wetting everything. If this is the case there is plenty of comment on this in old threads on this forum. Good luck with sorting it out, whatever the cause.