The 3 bees visible near the hive entrance at the end of my last post are likely foreign bees begging for acceptance in my colony. I saw more of these every day, always a few of then at the entrance, with guard bees fighting them, biting their legs, wings, dragging them back. They don't fight back. Their hair appear normal, particularly on the thorax, so I don't think they are robbers.
When one of my bees is on top of one of these foreign bees, then the difference in size and color is more evident. I have seen a few managing to enter the hive, but they were dragged back. They usually end up dead near the entrance, killed by the cold of the night.
What makes bees leaving their colony? I noticed these bees since June 19th, but it may have started before that.