L293D drives motor without supply voltage

Hi,
I'm experimenting on using a L293D IC to run a Nema17 stepper motor, but the IC doesn't seem to rely on the supply voltage to run the motor as it still goes when I plug it out and I can't see any current drawn from my power supply. Have anyone else had this issue?

Thanks!

If you don't provide a motor supply to the chip it takes power via internal circuitry from the 5V logic supply which
it is not designed to do. This can fry the chip and overload the logic supply. The chip assumes the two supplies
are both present if either is I suspect.