Does it matter if he's away for a 'good' thing or a 'bad' thing? In terms of good things, perhaps something as simple as Hannibal following after a woman- perhaps a married one, to explain why he's keeping it secret/not sending word. Or one with enough money to provide alcohol/drugs and he's been distracted.

In terms of bad things, maybe he owes someone money (or, alternatively, won too much money from someone gambling) and is in hiding from them? He could also have gotten sick (/injured/on a days-long bender) somewhere away from his normal places and is having trouble getting back.