I think the thing that I like most about the Miss Marple mystery solutions, as compared to Christie's other detectives, is that they're frequently very domestic. By that I mean that they involve things that professional investigators - who were, at the time, frequently middle-class men - are simply oblivious to. Such is the case here. The solution involves a disguise, which is device that Christie tends to overuse, in my opinion, but the disguise is one that's designed to go unnoticed by anyone who might reasonably investigate the case. Unfortunately for the perpetrator, Miss Marple is an unreasonable investigator and so it doesn't get by her... but then not much does.