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You know what I have done in the past and actually have set up right now is my second bedroom also seconds as my office, so I just have a webcam on that computer and either set it to record on motion or use a remote viewing/recording program. Don't know if putting a computer in there is an option for you, but if you use the right software, it's all but undetectable even if they decided to "snoop" on your computer. I've caught my niece, sister-in-law, good friend, and even my other friends sister that way! Just make sure the webcam has the ability to run without the light (or break the light) and you're good to go.
Other than that, you can build your own spycam easily. If you can't justify having a computer in the room and have access to a laptop, hide the laptop in a drawer and build a structure for the webcam that wouldn't be weird. Some suggestions could be a lego (if you have children), a box that is sitting there, a backpack or other type of bag, or a book (but be careful with that one, people love to pickup books!) etc.
If you absolutely cannot go with a computer in that room or don't want to trouble yourself with having to "hide" it, there are more options than the one posted above. A Pen camera or similar might work for you (http://www.amazon.com/s/ref=sr_st_review-rank?keywords=Pen+camera&qid=1398673438&rh=n%3A172282%2Cn%3A502394%2Ck%3APen+camera&sort=review-rank) that first pen one at that link is pretty interesting and might work, if you don't need remote viewing.
