"I'm sorry, I didn't mean to startle you." Bailey apologized when she saw Peter jump. It was perhaps not a good idea to scare the potions professor when he was brewing something - or when he was holding one of his pet spiders. It hadn't been intentional though.
He was right though that things needed to be done - and with teaching all week long, the weekends became the best days to do long brews. "I suppose it is a good day for such things." It certainly made sense.
As for why she was here, "I sometimes find Lia working even on the weekends. I tend to look for her here first. We don't have any specific plans, but I wanted to take advantage of the fact that it is before curfew to get out for a while. I'll have all night to work on my homework."