How can we be charged days later ?

Two weeks ago my son and I was at costco. I was in line with my granddaughter waiting to pay for an item I had to go to the restroom so I had her wait in line while I went . On my way out the restroom I pasted the office a man stop me , he pointed into the office toward my son and ask me if he was my son. I said yes and what was wrong. They said there was a problem with a blister pack that a camera was missing and they saw my son put the pack under something in the store and the camera was missing. I told them that that case was in the cart when we got it and to go ahead and search my son ( hes 24) they search him and found nothing. they apologized and we left the store. Then two days later the police came to my house with arrest warrant for both of us. How in the world can they search my son find nothing let us go only to arrest us days later ?