The prescription drug use (even if she goes to several different doctors) and is addicted to prescription drugs, would NOT account for where all the money is going. Going thru large amounts of $ could indicate abuse of drugs such as crack, meth, or heroin (I'd suspect crack first). How do you know for sure? You get a multi-drug urine test panel (I know you can get them over the internet and I think you can even buy them at walmart) and you catch her by surprise and ask her to pee in a cup right then. Can't force her to, but if she won't do it, what would that tell you? If she does, you'll be able to tell right away if she's positive for cocaine/crack, or amphetamines (crystal meth). heroin would show up as positive for opiates - but I'm not sure if ambien would trigger any positives, though.
Stop wondering and urine test her. The not knowing, the searching thru her stuff, the guessing - all of that is crazy-making. Once you know what is going on, you can take action (or not) but at least you are dealing with facts and not supposition. Then, if it turns out to be drugs, come back here and we'll help you out with sorting thru everything again.