Can someone hypnotize you without you knowing?
Yes, no, maybe. I am sorry to start out that way, but it's really tue. Give ma chance to explain.
The problem really lies with the definition of hypnosis.
There's what I do in my office, where there's a recognized "induction" to create an environment where hypnosis can happen. Then using my training, I use certain techniques to advance that state until you enter into and remain in the state that we call "trance", which is the working space of hypnosis. Think of it as kind of like "the construct" that Morpheus teaches Neo to use in The Matrix. It's big, infinite really, and we can fill it with pretty much anything we can imagine. I think we can all agree that is "hypnosis".
However, imagine that you are tired, after a long day of holiday shopping and you are looking for that one last, perfect, gift. You are running out of time, the mall is closing soon when you encounter the nicest salesperson you have ever met. She seems super knowledgeable and she really relates to your dilemma. She's got some great suggestions and she gets you to imagine the scene when you give the gift she has to offer. She describes the joy in face of the person you are giving it to and she tells you a story about how she once got something just as special and how she still cherishes it and thinks goo things about he person who gave it to her. You fine yourself buying what she has to offer, though it's not something you had even thought of before you encountered her.
Were you hypnotized by her? I've had sales training. Heck, I have conducted sales training and I have to say that many of the techniques are mighty close to some of the training I have in hypnosis. Some people call this "waking hypnosis" or "covert hypnosis".
With that in mind, yes, if you consider influencing you to be part of the definition of "hypnosis", the you are influenced or "hypnotized" by advertisements, media, friends and even salespeople all the time.