On a power supply board for an X-ray sensor (PAN Sirona) we are getting -9v from an LM137 fed by an LM2672 generating -12V from +5V. The circuit is likely like that (it is not easy to make sure it is exactly like this since it is a multilayer board):
Is there an application note that specify the usage of LM2672 to get a negative voltage since the datasheet doesn't mention this kind of usage?
It's a well-known configuration:
AN-1157 Positive to Negative Buck-Boost Converter Using LM267X SIMPLE SWITCHER® Regulators | Texas Instruments
Might also be called a bootstrapped regulator, as it's essentially driving its own ground reference; but the power switching topology is the same as the archetypal inverting buck-boost.
Your reading of the PCB layout is pretty close; it's mainly missing the input and output filter capacitors, and R1 goes to FB (as shown, it's just loading output to GND, no effect on feedback).
