I presume being collapsed would account for no signal.
Khafre's pyramid was built on a natural rise, and the sides are a sharper angle, to make it appear larger than Khufu's, but it has less than half the mass. The original entrance was rediscovered in the 19th century, but the broken lid of the sarcophagus was on the floor, left there by the tomb robbers, perhaps/probably in antiquity.
The limestone casing stones were stripped by Ramses II "The Great", for use in the construction of a temple elsewhere. Ramses II had a habit of having older pharaohs' images repurposed, and with Khafre, a number of the (at least) fifty-two life-size statues were recarved with Ramses' cartouche. He did the same to King Tut's colonnade at Luxor, but thanks to poor lighting near the ceiling (which is now long gone), one of Tut's cartouches were missed, rediscovered during the modern day recording of the inscriptions there.