When I click on the envelope icon, I see no activity for today. If I then click on yesterday, I see correctly the activity for yesterday. But when I then click on today, I see activity for today that was not there two clicks ago.
I see this in meta and Area 51 (don't have enough recent activity on other sites to know about them).
This is something I can consistently reproduce. I use Firefox (don't know if that is important).
This is probably related to (but not the same as) this defect: Recent Activity Area of my Profile is bugged - “Today” and “Yesterday” sometimes don’t work
Update: here is a directly related issue which I think is a bug: click the envelope icon then click reputation. "Today" will be highlighted but only recent information will be shown. But if you click on "Today", then Today's info will be shown. This is different from the above because, in this case, "Today" is highlighted, implying that info from Today is displayed, which is not the case.