I suspect, as usual, Frank is correct.
It appears to be some sort of pyroelectric sensor.
Although, the window filter seems to be composed of a different material than is normally employed in most sensors found in PIRs, perhaps indicating the device maybe tuned to a different wavelength range.
Perhaps a Near IR flame detector? Or gas detector?
It also appears remarkable similar, apart from the window filter, to the pyroelectric sensor in the PIR detector below:
Which also has three leads, by the way.
Although, again the window filter of your device maybe of a different material, possibly indicating the device maybe intended for a different purpose.
muRata manufactures a line of similar pyroelectric sensors:
muRata LEAD TYPE (IRA-E Series, IRA-S Series)
And the following device from another manufacturer:
BigDog