Fun fact: Incense was historically used for funerary purposes. It hides the smell of decaying bodies. It's also used in some churches, pilgrims can be very smelly. The egyptians used frankincense for mummification.
The smell is probably from the sewer btw. It's possible the water level in one of the sinks or toilets is too low, but your grandfather doesn't notice the smell anymore. It may be enough to simply run the taps a bit.
Or your grandpa murdered someone and hid them under the floorboards.