Lieutenant Anderson tells the women that he'll cover the house from attic to basement, which must imply he'll be there all night because when he comes in after Judy's murder, Cornelia asks him where he was.
However, Anderson also told Dr. Wells the same night he would check the doors and windows and then be back in the morning. Why would he risk saying two different things to the doctor and Cornelia when they might consult each other?
He lied to Welles because he (correctly) suspects that Welles is copycatting him as the Bat in order to search the house for the million and throw suspicion off himself.
shareI don't know that the doctor is quite that clever because it's not until the final showdown in his office that he actually sees the Bat, so he doesn't know what the Bat looks like, so he couldn't really copycat him at all.
Besides, we don't know how much time there is between when Anderson leaves and when he returns after Judy's murdered...there could very well have been enough time for him to conk out Warner, put on his suit, sneak into the house, chase Judy down the stairs and kill her, run out, ditch his costume in the bushes, and come running in looking all innocent.
That is what Anderson did he knocked Warner out bc Warner was following him to his spot to put on his costume,
shareHis first mistake and the first giveaway it was him was when he was in marks office and he's ranting about how all his money is tied up in that and he's finished if it doesn't get found. So I knew then it was him, and after that every other scene with him just confirmed it, plus it's never who the obvious suspect is made out to be!
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