Without a fiber connected to the Fiber OneShot™, the length reading should be 0 ft or 0 m. It’s very unlikely, but it may read 1 ft or 1 m. You can "calibrate" it to 0 ft or 0 m.
Let’s make sure you are the right screen first. Press TEST so that the length reading is reported. It should of course show 0 but in this case it shows 1.
Press and hold down 
for 5 seconds to see 0CAL appear.
Press and hold down the 
for one second to start the calibration process.
When you see done, press TEST to verify that it now shows 0 ft or 0 m when no fiber is attached.