To factory reset a Polar S725x (possibly S725 or S625 and others?) follow the following steps:

1. Using a paper clip or some other small, pointed tool press and release the RESET button which is located between the UP and DOWN buttons.
2. All the LCD segments on the display should light up.
3. Press and release the bottom left (STOP) button followed by the top right (UP) button. You should now see “Monitor S725” on the screen.
4. Using the UP and DOWN buttons, scroll until you see “Default SET” displayed.
5. Press and hold the big bottom middle (OK) button for a few seconds. This should cause “Default SET” to flash, keep holding the button until the display resets to that shown in step 2. Release the button.
6. Press and release the big bottom middle (START) button

Factory defaults should be applied and any user settings should be cleared.

It’s a shame this procedure is not documented in the Polar S725x manual.

Thanks to “Wayne@PolarUSA”‘s post which gives just enough info to figure out the specifics needed to perform the reset.