I'm assuming that 2758 is the reputation after the upvote and accepted answer. In other words, that your reputation when you left last night was 2733. Subsequently, after identifying that information, you saw your reputation drop by 15 points to 2743.
Because the drop is 15 points, the most likely scenario is that an acceptance vote on one of your answers was retracted. You won't notice this on your envelope activity report unless that specific acceptance that just happened was the one revoked. The nature of this action is that it subtracts by removing the original gain, not by adding a negative gain, so this is reflected in both a reputation audit and your envelope activity report by removing the original vote.
There's also the possibility that a combination of question and answer upvotes equalling 15 (1q1a or 3q) were retracted in the same manner.
Naturally, though, if you check your reputation audit, and you see some negatives around that time, that would be the cause. But if you find that neither the reputation audit or your envelope activity report show any recent loss that would result in this, yet a visible loss of points occured, then it will have been caused most likely by the retraction of votes.