If you look at the patron record it says an sms was generated at the time of cki. But it doesn't actually go out until after day end runs and then only between 9a- 7p. So basically the morning after the item was checked in and put on the hold shelf.
So most patrons will only get sms notices at about 9a every morning! Unless it is a general message and those go out within about 10 minutes but only between the hours of 9a and 7 p.