I put bee hotels up in my garden for the first time in spring 2014 & had an enthusiastic response from a number of small bees Ian Beavis kindly identified as Hoplitis claviventris. I was photographing one of the females as she was sealing her nest, and followed her as darted in and out of adjacent empty tubes collecting what I think was wood dust and placing it in the middle of her masticated leaf seal.
I've laid out the sequence in two rows, left to right. Once she had her white daub in the middle, she didn't add any more. I can't work out why she did this, and it echoes what Ed Phillips observed with his Pemphredon? wasp, posted earlier on this forum. Perhaps it was just a finishing touch!
(North Cumbrian coast)