Normally there should be no reason to reset a device. Nevertheless, here is the procedure.
The rechargeable sensors, namely the locators and the push button, can be reset by putting the device into flight mode and back into normal mode. To put a device into flight mode press and hold one of its buttons for at least 10 seconds. The LED will blink fast upon releasing the button indicating that the device has changed the mode. Once it flight mode it is asleep and will not send any message anymore. To wake it up from flight mode repeat the same procedure (press button for 10 sec.). It will then be reset and try to join the network again.
Note: Newer firmware versions indicate the wake up from the flight mode with a solid LED for a couple of seconds instead of fast blinking do differentiate between falling asleep and waking up.
Sensors with replaceable batteries
These sensors have internal capacitors. Therefore, it is not good enough to briefly removing the battery for resetting the device. You have to leave the device without a battery installed for a day to make sure it ran empty and thus got reset. Alternatively, you could short-circuit pin 6-10 as shown in the picture below if you want to reset it immediately.
You can see how to open the device here.