I have recently heard a lot of people (most of whom have learned English as a second language) use the phrase "I slept at 9 o'clock" instead of "I went to bed at 9 o'clock" or "I fell asleep at 9". I was wondering if there was any idea as to the origin of this phrase.
It doesn't sound at all odd to me.
While laughably untrue, this is grammatically correct, and the sense is clear. If you look at the native language of a non-English speaker who says it this way, you'll probably find that the typical way of expressing the sense is I slept rather than I fell asleep or I went to bed, that's all. (I know this to be the case in Chinese, for example.)