My first reaction to your post was, "Better a condom than a positive pregnancy test".
I agree, it's possible she's just being prepared "in case" it happens...nothing wrong with that..that's the responsible thing to do as a young woman.
I tried to look back at my own teenage years and what would have been the best way to handle ME when I approached these delicate situtations with my own teenagers. I was 15 years old and used no protection. I did go to a local clinic and get birth control pills soon after, but I never did discuss it with my parents. My parents were the age of most people's grandparents and they were overly strict with me as it was, the last thing I could ever have told them would have been about having sex during my teen years..although I know they figured it out when I became pregnant at 17. I am not sure talking to my parents would have changed a thing, I would probably still have had sex...that really is a personal decision for all of us no matter what the age, and it's a difficult thing to discuss with your parents..especially if you are a teenager, even when you have the best of relationships.
I think truly, we want to believe our kids will abstain, or if not they will come to us with these things, but in reality, these are not the things kids want to discuss with us, any more than we wanted to talk to our own parents about sex, boys, relationships..lol. I encouraged my own kids to be open with me about anything..and they were for the most part, but I also know in retrospect (they are all adults now)there were alot of things they did not share with me. I have come to the conclusion that this is by natures design somewhat, part of that rebellious process that starts the movement away from being mom's child to a young adult.
First, I would be glad that she seems to be taking some responsibilty for both disease prevention and pregnancy prevention...so many don't, thinking it won't happen to them. Kids have been having sex since the beginning of time and there's no reason for us to think that changes just because they casually say they will inform us as parents. Let's face it, it is not an easy subject to approach for parent or for the teenager.
So, if she's having sex. The best thing a parent can do is make sure she is protected and to let her know you are there if she needs anything..to talk to, to assist with birth control, to give the benefit of your experience or to just listen.
I think my approach would be to let her know honestly you found a condom..you don't have to say where you found it..and then thank her for being responsible. That will keep it from feeling confrontational..and will hopefully lead into a more indepth conversation.
With my kids, I tried to remember that these are adult actions, and I needed to approach them with adult discussion, even if I was in a panic about the entire thing. Deep breaths, calm approach. The best thing for her, and for you is an atmosphere that will promote easy discussion. It's not easy to do when all you really want to do is lock them in a closet until they are in their 20's..lol.
I would definitely tell her you would like for her to see a doctor and discuss the many options in birth control. (Condoms only work if they use them.). I would also explain to her that once you are sexually active, there is a whole lot more to keeping yourself healthy than just preventing pregnancy..and let her know she will need gyno care on a regular basis, like all adult, sexually active women do. Volunteer to set up an appointment for her, give your recommendation for a doctor you like and volunteer to go with her to the appointment.