Is it okay for an empolyee to be subject to search when they are off work and shopping in the store he/she work for.

While seemingly unfair, it is probably legal.  This is true because most employment arrangements are what is known as "at will".  This means that basically the employer can hire or fire someone for any reason or no reason whatsoever, as well has increase/decrease salary/hours, promote/demote, and generally impose work condition that they deem necessary.  In turn, you can work for an employer, or not, your choice.   So as long as this action does not violate a term of your employment (union/employment contract, stated company policy - whether or not previously enforced; or discrimination was involved) it is perfectly permissible.