I have heard from my friends with similar cases as yours.
The maids took out some of daily necessities (eg. oil, rice, cleaning detergent etc) from the employers home and then RESELL them either to the employers or to the outsiders during their holidays. When she accumulates a full bottle of oil or cleaning detergent, she can simply give it to the employer as a brand new one and claim money from the employer when she is asked to buy them.
Just share what I have heard, maybe your maid is not that bad.
Wa, unbelievable!! They are that tricky and that bad!!! I think they will use quite a lot of oil for cooking, same as my previous Thai maid. Next time when my new maid come, I will also keep track of the oil and detegent.