I'd suspect that there is some emotional issue that is occurring because as you said, she's using the bottle in private, at home. She cant stand out because there's few people to see her and she's obviously trying to conceal the fact that she is doing it. She's doing it at night presumably because everyone else in the family is asleep. I don't think that it's simply because she's wishing to experiment or wants to "be smart" with you.

As for sleeping all day, that may be a sign of something or it may not, it depends whether or not she goes out with friends, does things around the house, etc... . How late is she sleeping until? Many teens (myself included) tend to sleep in late (i.e. until noon, maybe a bit past), although when classes start that changes. But if she's constantly sleeping, such as until dinner or so, then there may be an issue to address.

Her sleeping very late in the day may have something to do with the bottle. If she uses the bottle a lot during the night to a point where she doesn't sleep much, then come later, she's tired and sleeps in.

You could ask her about it or you could also put the bottle in a different place (and any other baby bottles you may have). If she ends up frantically searching for it, then you know something is definitely up or if she uses something else in place of the bottle, then you also know something is up.