As I removed one of the sides of the bee hotel to examine the progress of the Red Mason Bee nests I found that one of the cells had been occupied by a mature Mouse Spider female, Scotophaeus blackwalli, in her silk cell, looking like she will be ready to nest soon. Although she retreated to the bottom of the groove, she was still there the following day.
These spiders wrap their eggs in round white silk sacs under bark or in crevices in and around houses, and the female sits by the nest, guarding it. These spider nests are not easy to observe without disturbance, as they may fall apart as they are uncovered, or the female might flee. I am therefore quite pleased with this new guest, as, if she decides to stay, I should be able to watch her nest with little disturbance.