Hello,

I see allot of great answers to your inquiry.
I do not have an answer to the cause, but
I do have a gut that tells me the robbing has
probably been going on longer than you believe.

A hive being robbed can sometimes appear, 
-even to experienced beekeepers as a 
colony thriving with bee traffic.

 >>however the local dearth
effectively started two weeks early.  Last week I noticed some robbing
activity on both hives (bees trying to enter but not knowing where the
entrances were, tussling at the entrances) and reduced the entrances.
---

In the early stages and during the frenzy of robbing,
with screened bottomboards, often, foragers which are 
newcomers to the scene do not yet know where the 
entrance is, so you will see bees testing cracks and screen
bottoms at this time, while other robbers who have 
learned the entrance are using the entrance 
and may appear to be normal bee traffic.

Small colonies in very large hives often abandon
defending the entrance, in favor of defending 
the core nest.  This is devastating to the colony
because it can be overwhelmed easily once the 
entrance is breached.   And it will begin to look
like normal bee traffic, because the fighting is
going on the inside. 

 >>Very
mild robbing compared to some instances I have seen in the past.  Robbing
stopped but the following days forager traffic dropped off sharply.
 Yesterday found both hives to be completely empty.

Once the entrance is learned, they will use that entrance and 
testing will subside.  The forager traffic which dropped off,
was IMO, the robbing event, which subsided abruptly.

Dont believe the German proverb.
Nasqult a chunche, varlearch ar laer

You will succeed in the future. 
Best of luck to you!

Best Wishes
Joe Waggle

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