What's up with these tiny flies?

Since a couple of days ago I'm finding several small, very tiny flies in my apartment.  It's not an infestation, just one in this room, another one in that other room, and so on.  But a few days ago I didn't have even one of those, so it surprises me.  Today I killed two of them, and I think there's at least one more left flying around.  WTF?  The apartment is as clean as usual, I'm not leaving food outside, and I throw away the garbage every night.  I even spray the lower edge to my door from the outside twice a week.