SailfishOS version Vaarainjärvi (18.104.22.168) brings the following Power key shortcuts to Sailfish OS. Both of them affect the screen lock. It is not possible to disable these shortcut commands.
Screen lock (and the complete device lock) is explained in this article.
Immediate screen lock
Double press the Power key quickly when the display is on.
The display turns off and the screen lock goes on immediately, regardless of the delay set for Automatic locking in menu Settings > Device lock. (*)
This implies that the touch display must be unlocked by typing the Security before using the device apps the next time.
(*) There is an exception (which may be a bug): if the setting is "Not in use", then not even this manual action cannot lock up the screen.
Double press the Power key quickly when the display is off.
The display turns on and then the device directly presents the Security code dialog.
The purpose of this feature is to make it faster to get back to using a locked device without compromising the lock utitility.
NOTE: This feature does not work with the Xperia devices, unfortunately.