I saw lots of activity in my 3 frame nucs very early the day after I made them - and I knew that was too many bees to be native to the small nuc. It was too early for an orientation flight too. Robbers!!!
Open feeding triggers robbing behavior, IMHO. And it triggers odd behavior from the bees - out very late, or early, drawn to flashlights since many caught out after dark, and not something I want to deal with. Just sayin'. It can be a solution to a problem for a given situation, I'll grant that. Can't picture that being the case for me ever tho.
I have a 3 way mating nuc, entrances are 1" holes on front, left and right close to bottom, center hole close to top. I installed mesh robber screen- a fancy name for what I actually did, which was put 3 nails around the entrance, force metal screen (like for a screen door) on so the entrance was covered, and secured with clothespins.

I used the screen to make a tunnel - the mouth of the tunnel had to be more than 1" from the entrance hole, or the robbers could swoop in. It was OK if the tunnel was around 1/2 in tall and wide, it didn't need to be just one bee in width, as long as the mouth was not too close to the hole. The robbers are very persistent at flying at the hole (covered by mesh), and can find a direct path in, but don't seem to take a path in that involves walking on the face of the box more than about 1".
The screen tunnel-mouth for the top hole was aimed up and a bit to the left. The screen tunnel-mouth for the left and right bottom holes was aimed at the outer corners of the box. I got 3/3 back mated, which has not been the rule this summer. :/
Hope this helps. So many little things matter so much for successful queen rearing!!!