Hi
If I was you I would just leave it, note it and disscuss with the cleaner, if she is good then it may be best to hang on to her. I had a cleint once who was absolute in her claim that one of my girls had stolen a blouse, she just new! I just asked the girl if she had seen it, no was the reply, i then asked the customer to have another look, and she came back to me having found it and was very appologetic.
it could be a mistake on the customers part, if your cleaner did take it then she knows your on to her and she would be foolish to do it again, just keep your eye on her.
As for girls/staff taking cash payments, my girls regurlary take arround £400 per week in small payments and I never have a problem, and remember there are more tempting things in customers homes as well as cash for them to take, if you can't trust them with collecting money then don't let them clean homes is my moto.
kind regards Phil