This is what I do for a living....the type of business I run and have had up to 6 people working for me. My advice is to let the business owner know if she works for a company. If she's a solo operator, then I'd ask for my key back if she still has it, and that would be the end of it. This sounds like someone with a problem to me. She likely saw the cameras and still was going through your medicine cabinet and drawers looking for prescription drugs. I tell everyone who works for me that they should always assume that they are being surveilled at all times, because they likely are in many of the homes we work in.
I'm wondering if you checked her level of experience, references and if she had insurance? Those are all of the things that I tell people to look for when hiring professional pet-sitters. Sadly, with apps like Wag and companies like Rover....they will literally let anyone work for them and so I hear about more and more of this. And with social media in general, anyone can just go on Facebook and say..."hey, I'm a pet-sitter, or a dog walker...hire me!" I've had my business in my area for almost 15 years and with that comes a lot of experience in hiring and trust from the community I've only had this happen to me as an employer once, and it was early on. I hired a friend of a friend who came with good references, but she was tipping from being an alcoholic into pain killers. This customer had cameras and she knew it....and still couldn't help herself. I confronted her...gently, because anyone who has gone down this road is struggling. She admitted it. I counseled her to get some help. I knew the customer really well and told her the story....no charges or anything were filed. I sent the customer a big gift basket with things for her pets, and I still have that customer to this day.
I'm sorry you feel violated, but I completely understand that. I would say to look for someone who has been doing this for at least 10 years and has a good reputation. Anyone working in my world that long is not a thief....because in this world now where there are literally cameras everywhere, we'd never make it that long

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I also know people who have hired plumbers, electricians...etc, and are "home" but don't think to take things out of the medicine cabinet...etc. Prescription pain killer addiction is a huge, huge problem in our country. If you have it in your home....my advice, is to lock it up somewhere. It's just the safest best all around. I mean, it's so bad that local newspapers won't list the cause of death in an obituary, because if they print that a person "died at home surrounded by family after a long illness"....to an addict, that says..."there are drugs in that house." So...better safe than sorry all around!