The comparator circuit that turns the oscillations from the crystal oscillator into a digital signal has an automatic threshold adjustment that aims to keep the duty cycle at 50%. If there is too much high time, then the compare level is raised. If there is too much low time, then the compare level is lowered. A side effect of this control loop is that it will keep the output of the ECO oscillating even if the input is DC. The frequency that the ECO oscillates at is not specified, and in fact varies greatly -- but the ECO output will oscillate.
The rate of oscillation is affected by noise in the system and depends on the voltage level of the input. From observation, if the two inputs are shorted together the output frequency has been measured to be in the 20kHz to 40kHz range. If the two inputs are not connected to anything, the output frequency has been measured as low as 5 kHz and as high as 100kHz. Also from observations if either or both inputs are shorted to Vdd or Vss the output frequency has been measured as low as 1kHz and as high as 100kHz.