I’ve tried using Face ID in the dark on my new iPhone X and I can very clearly see the infrared flood Illuminator light up as well as the infrared dot projector. Is it possible for humans to see infrared light?
They typically emit very-very-near infrared, close enough to red to be detectable by a regular CMOS camera.
But the IR LED doesn't emit a single wavelength, there will be some lower level of emission at shorter (red) and longer (more infrared) wavelengths. In the dark your eye is quite sensitive to very low light levels and so can see the small % of light at the shortest wavelength emitted by the infrared LED in the phone.
Perhaps you could be seeing some levels of infrared light, because it is very dark. This means that you could see some low wavelength light due to your sensitivity of your eye(as Martin said). It is not likely that the iPhone X would be emitting red light, as that would seem to me to be rather useless. Hence, it is likely that you should be able to see some very dim infrared light, but it should be really dim. Since you said that it was very clear, it might actually be red light.