It was strange. Papyrus usually followed the exact same schedule every day. So, when he didn't show up to recalibrate his traps (with her keeping him company), True knew something was up.When she called Undyne, she said he didn't show up for training either. However, she also said it probably had something to do with his "cycle", whatever that means, and that she wasn't too worried. Apparently it's a yearly thing.
Still, the thought that something was wrong kept pestering her. So, of course, she had to go pay him a visit.