How did Dev eventually learn the truth that Sai Baba had, in fact, attended his Udyapan ceremony as a Sanyasi?

Dev discovered the truth through a chain of communication detailed in Chapter 40. Feeling disappointed and deceived, Dev wrote a letter to another devotee named Jog, expressing his sorrow that Baba did not keep his word. Jog then took this letter and presented the matter to Sai Baba directly. Baba, in turn, revealed the entire truth, explaining that he had indeed come as a Sanyasi with two others, just as he had hinted he would. He rebuked Dev for not recognizing him. Following this revelation, Jog wrote a detailed letter back to Dev, recounting Baba's explanation. Upon reading Jog's letter, Dev's eyes overflowed with love, and he felt deep regret for having doubted Sai.