The next morning, Saul prepares to return home. Now, remember, Saul thinks that Samuel was just speaking in jest about being king, so Samuel tells him to send his servant on ahead so he can tell him the Word of the Lord. 

Then Samuel took a vial of oil, poured it on Saul's head and kissed him and told him that the Lord had anointed him to be captain over His people. As a sign, Samuel said, When you leave here you'll meet two men by Rachel's tomb, and they'll tell you that the mules you went to search for are found and that your father now worries what happened to you instead of the mules. Then when you leave there, you will meet three men, one carrying three kid goats, another carrying three loaves of bread and one carrying a bottle of wine. They'll greet you and give you two of the loaves of bread.  

When Saul turned to leave Samuel, God gave him another heart, and all the signs that Samuel spoke, came to pass that day. 

If you will recall the method by which they determined that Achan stole the accursed thing from Ai, bringing all of Israel, first by tribes, then by families, then by households, then man by man (this was probably done by casting lots or perhaps by Urim and Thummim) until it was revealed that Achan had committed the transgression. This was also how it was shown to the people that Saul was the anointed one to be king of Israel. First the tribe of Benjamin was chosen, then  the family of Matri, then Saul, but Saul was nowhere to be found. So they asked the Lord where he was and were told that Saul had hidden himself among the "stuff." (One might assume the stuff was baggage, since people had come from all over Israel to sort of inaugurate Saul anointed king.) 

Remember earlier in our story, the Bible said Saul was (to use the exact words) "a choice young man, and a goodly: and there was not among the children of Israel a goodlier person than he," so one might assume that he was humble, and that was why he had hidden himself when all this "to do" was going on about the new king.