Barry, have you witnessed them come out and cover the hive or have you only walked up and "discovered" this after it has happened?
The reason I ask is that one evening/night I got smart and decided I would mow the grass around the hives. It was dark, bee's don't fly in the dark...right? I had a red headlamp I would wear. Great plan!!!! I hopped on the trusty old John Deere and rode down to the hives. I made the first pass behind the hives (they're laid out in a straight line). The second pass was in front of the hives. I turned and came back around behind the hives and looked at the first one I was beside....there was what looked like a thick bee carpet all over the hive. As I proceeded down the row the other hives had followed suit. As I turned and started down in front of the hives for the second path I noticed a bee, er, uh, bees (plural) crawling on my legs in the dark. Hmm, nobody told these bees that they weren't supposed to fly at night!!!! It was a very CREEPY sight/feeling seeing all those bees crawling on the outside of the hives and knowing there were some crawling on me...and all this by the eerie glow of the red headlamp.
I shagged it back to the house!!!!
I wonder if something isn't aggravating your bees.... A "fume" floating in to them...I wondered about my mower's exhaust. A skunk visiting though I'm not sure they would cause such a thing...I know my mower probably created a vibration that they felt.
Something causing them to explore/patrol the outer surface of the hive.
Ed