In IL, a "shopkeeper" can definitely detain someone:

> (a) Detention. Any merchant who has reasonable grounds to believe that a person has committed retail theft may detain the person, on or off the premises of a retail mercantile establishment, in a reasonable manner and for a reasonable length of time for all or any of the following purposes:

> (1) To request identification;

> (2) To verify such identification;

> (3) To make reasonable inquiry as to whether such person has in his possession unpurchased merchandise and to make reasonable investigation of the ownership of such merchandise;

> (4) To inform a peace officer of the detention of the person and surrender that person to the custody of a peace officer;

> (5) In the case of a minor, to immediately make a reasonable attempt to inform the parents, guardian or other private person interested in the welfare of that minor and, at the merchant's discretion, a peace officer, of this detention and to surrender custody of such minor to such person.

https://ilga.gov/Documents/legislation/ilcs/documents/072000...