If you received a check from a company that you did not sign up with and provide a SS#, it is almost certainly a SCAM. Legitimate mystery shopping companies do not pay you before you complete a job, they pay you afer you finish the job and complete a report for them. And legitimate mystery shopping companies do not send checks that have to be activated or ask you to cash the check, keep some money and send some money somewhere else. Since you provided no information about what you received, who you received it from, or what instructions they sent you, it's not possible to comment on your specific situation, but I'm betting SCAM.