I still think it's the cap and/or it's coming from that vent hole.
There could be something going on with a blockage of the internal filter, or something else causing the level to be higher than normal.
- cleaning it up
- replacing the O-Ring
- insert a Q-tip into that vent hole you can see in the cap and break it off so that there's still enough stalk to pull the Q-tip out. This allows it to vent but dramatically reduces the changes of liquid coming out.
From what I see, I don't think this is coming from anywhere else upwind of the reservoir - I see it all the time on BMWs.