For KDE4 was be kbluelock program, that will automatically lock / unlock screen, when bluetooth device is near to computer. This was very useful in old KDE, but this app is not ported to KDE5. So, will be good to have same cool feature for KDE5 too!
I have found cli solution that can automate bluetooth proximity tasks: https://github.com/Thor77/Blueproximity
*** This bug has been marked as a duplicate of bug 266829 ***