I read about that this morning. Very interesting.
Sometimes there were faux tombs to discourage the tomb raiders. Herod is buried SOMEWHERE over there, to be discovered SOMEday...or his tomb has already been ransacked.
Already ransacked is my bet. Possibly by his own family.
This could be his tomb - after the original was looted. Or, IIRC, Herod “The Great” was much hated at the time of his death. Perhaps they put him in a modest tomb to hide him from folks who would want to desecrate his grave.