Because he had them the whole time. Why would EJ and AJ inspect his backpack when they picked him up? He said the (fictitious) robbers gave it back. I think he had his electronics with him the entire time. He barely spoke to AH when he got home. He probably stashed his laptop/ipad under the bed, under blankets in a closet, in the safe...somewhere in the apt, and retrieved them whenever leaving the house. (Kids do this all the time when hiding their "contraband" from their parents.)
They didn't find any of those things when they searched the apt Friday, which was concurrent with his interrogation/confession, because he had these things with him. I think he always carried these things with him. People who have things to hide take their laptops with them, after all, they're mobile devices!l.

I know you question the




, but I do believe he had some or something on his computers that he didn't want his wife to see. Info about secret trysts with other people? Who knows?
ETA: PIM, Amanda said she is confident he had the BP when returning home. JH also says the robbers (there are no robbers) gave it back, ergo, he had it with him. The Judd's didn't mention it because why would they? And maybe LE did ask the Judds but it's just part of the details omitted from the affidavit. No one would know what was in the backpack on Tuesday night unless they searched it. No one ever searches me when I come home or when they give me a ride. And I don't volunteer to show them what I'm carrying with me, IYKWIM.