Installing Robber screen

[Newbee]

So I got my Robber Screen from Brushy Mtn today (nice construction, looks pretty solid, wish the shipping wasn't so expensive...), do I have to install it at night/early morning, and shoo all the bee's inside, or can I just slap it on in the middle of the day?
What I mean is, from what I understand, bee's leaving the hive after the screen is installed, will be able to remember and get back in, but strange bee's outside the hive, e.g. robbing bee's, won't be able to figure out how to get in. Does that apply to bee's from this hive that exited prior to the screen being installed? Or will they be able to figure out a way back in?

Speaking of robbing, is there a description/definition of what robbing activity looks like?
Last year my hive was robbed out in dramatic fashion - it was pretty obvious when I went out there that something was going on. Big cloud of bee's dive-bombing into and out of the hive. Recently I've see what appears to be similar behavior by _some_ bee's, but not the big-cloud that I saw before, only a handful, maybe 20-30, and there doesn't appear to be any notice from the guard bee's at the entrance (which is already reduced). There are some bee's coming back with loads of pollen that are going in juts fine. Is robbing always a massive event, or do a handful of individuals attempt it as well? Wouldn't the guard bee's be able to catch them coming/going?
Thanks.

- Kevin

[LazyBkpr]

Ealy morning or late evening is best, it lets the leaving bees know something has changed... however, I HAVE slapped a robbing screen on in an emergency in the middle of a day. The bees will figure it out.
  The purpose of the screen is to STOP the robbing bees from ZIPPING in past the guard bees. Once the guard bees are overwhelmed the robbing snowballs, and that hive is done for. unless you can stop it in time and feed them back to where they were. The screen forces the bees to WALK into the hive entrance, and gives the guard bees a fighting chance.

[Bakersdozen]

Some signs of robbing to look for include a frantic behavior as they enter the hive.  Excessive numbers of bees coming and going when there is not a nectar flow on, could be robbing. Guard bees, if still alive when you are observing the action, will be fighting at the entrance.  If the robbing has gone on for a while, you might see wax debri at the entrance.

[Newbee]

Thanks guys.
Based on that description, I _do_ think some robbing is going on.
Guess I'll get out there early tomorrow and put it on post haste.
Thanks again.

- K