Leafcutter Bee

Over the past week or so, I have been finding holes in a seed tray of compost (with wildflower seeds) and couldn't understand what was happening. Then last week I noticed a leafcutter bee enter the hole with its leaf. It disappeared into the hole for over a minute then flew off and was back again with another leaf. The process continued - the bee would fly out of the top opened window in the greenhouse, do its work and then return with the leaf through the open door. Clearly, I needed to water the seeds - though I tried to avoid the area with the entrance hole ... a bit silly because the bee would have tunnelled through the compost! The bee soon moved to another tray and the process continued. But I still need to water the seeds. Has the bee abandoned the damp compost or is it using more than 1 tray?
I have found a shallow wooden box and filled it with compost with the idea that I will leave it so that the bee can build its 'nest', undisturbed. 
I don't know about the habits of leafcutter bees and am curious about why it is using the greenhouse seed trays. I wonder if the ground outside is too dry and hard. I took a video of the process and taken a screenshot of it, below.
Any thoughts please?