Jamieson, Fausset and Brown on Mark 13:23

23. But take ye heed; behold, I have foretold you all things--He had just told them that the seduction of the elect would prove impossible; but since this would be all but accomplished, He bids them be on their guard, as the proper means of averting that catastrophe. In Matthew (Mt 24:26-28) we have some additional particulars: "Wherefore, if they shall say unto you, Behold, He is in the desert; go not forth: behold, He is in the secret chambers; believe it not. For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be." See on Lu 17:23, 24. "For wheresoever the carcass is, there will the eagles be gathered together." See on Lu 17:37.