I agree, I don’t think he expected to get caught either. Possibly he had no idea there was such a thing as security cameras because he had several weeks to concoct a story about why he was driving around town. And maybe he did initially have a story? - he went from point A to B and directly home again. LE says no you were at C, he adds a little more to the story......but then what about D....or E grilling him until he breaks, leading to his confession.
Which brings me to another thought....Mollie’s dad. Did the family truly believe (or at least hope) she was being held alive or was that somewhat of a heartbreaking yet intentional ruse to trick the perp into thinking LE was totally clueless? I think of us here and all these past threads, 1000s of comments relating to the ambiguity of released information - was she taken while jogging or not, what was her route, what of dinner at her mothers, when was the Snapchat sent, what’s with was she home at 9:45 using her computer, questions, questions, so much confusion.... looking back, I now think that total lack of clarity in information being released was very intentional.