I believe Mrs BW's handbag exists both in our universe and in a parallel one. It is never where she last put it and when she needs it is never there. Then having searched the whole house from top to bottom, twice! It reappears in the spot that she claimed she last left it. My theory for what its worth is that there is another Mrs BW, slightly different of course, in another universe, but for reasons that scientists with much larger brains than I, have yet to work out, they share the same handbag. So when Mrs BW in our universe is not using her handbag off it pops to make itself available for Mrs BW in the parallel universe to use.
This explains why she says things like " My lipstick has disappeared again" and " I am sure I had a ten pound note in my purse".
Its not much of defense I grant you, but I never touched her lipstick.