I called the Citibank fraud department at 888-285-9696.  They confirmed it is a legitimate check relating to a Sears credit card.

I just called the number that Daniel Thompson posted and I too recieved a check from my old Sears card!! It was for quite a bit   too. I am glad I found this post!!!

Following up on my previous post -

I don't know how anyone else has been able to find out so easily that the check is legitimate, or get anyone to investigate it further (without resorting to calling the number printed on the letter from Citi).  I can't even get a single person at Citi to run the check number and tell me if it is legitimate.  I have talked to general Customer Service, the Identity Theft department and the Fraud department, and no one will run the check number or look into it any further.  No one is saying "We've had other calls about this Trilegiant issue."  They are all claiming they don't know anything and that they don't have the ability to see if the check is valid.

As above, on 5-30-14 I received a letter via U.S. Mail with an attached check dated 4-24-14 from "Citibank, N.A. Customer Service PO Box 6500 Sioux Falls, South Dakota 57117-6500". The Citibank letter stated sending a check regarding my previously cancelled membership in a credit monitoring program, provided by "Trilegiant" for program fees billed to my account.

I have never had an account with Citibank or Trilegiant. The letter list a contact phone number "855-841-4717" with any questions.

I got one of these letters also.  This is reported as a scam.  Notify the postal inspector if you receive this.

If you ever had an account with Sears, you should call them and check. I received the same letter, also thought it was fraud, and discovered that it was from a long-ago closed account with Sears.  A class action suit.

I also received the letter, do not have a citi account, and felt that it must be fraudulent.  My Theft ID company even called the 855 number for me, and it sounded suspicious too.  I then found the official number for Citibank and called to speak with them.  I realized that some cards, home depot, sears, Mervyns, gap, etc, are sometimes linked to the larger companies like chase and citi.  As it turns out - this refund is a class action suit from a Sears Credit Card.  I strongly urge you to do your own investigating.  But if you have ever had a Sears card, you should look into it further!  My check is legitimate.  

I also Googled the number for Citibank Customer Service and called them that way rather than calling the number on the check/letter.  Citibank confirmed that the check WAS legitimate.

I received a letter similar to all reports.   When I called Citibank fraud they told me to cash it and take a risk to see if it is fraud.  Really.

I was sent a check in the mail from citi bank I thought it was a fraud so i did not call the number on the letter I called citi bank and they gave me the fraud department. 1 866 771 3354  they ran a check to see if i had any accounts, and i did.   it was through sears years ago where i purchased credit reporting and that was what the check  was for.  and they looked and found i had a refund coming.  It is always good to be safe and never call the number on the letter.  always look up the number and call them direct. If it was a scam and you gave them the information needed to look up your account at the number given, your identity could be stolen.

Thank you, CWG40 and Tamianth.  I had a weird feeling about the letter and check -- even though the check is for a very small amount of money.   I don't recall ever having any kind of card or account with Citibank, but maybe Citibank acquired a bank I used to have a card with (long, long ago) at some point?  In any case, I have not done anything with this check, nor have I called the phone number printed on the letter.

To be honest, the first thing I did was come here to 800notes.  I could not find the 855-841-4717 number here right away, as there had been no reports of it posted yet.   Then I did some googling and typed in different phrases like "letter from citibank with check" and other similar sentences, which led me back to 800Notes -- and a few pages of reports from people who (apparently) received the same letter in March and April, except that their letters all had a different phone number (855-382-5097) printed in them.  Here is the page - http://800notes.com/Phone.aspx/1-855-382-5097.  The wording and purpose of the letters mentioned on that page seems to be exactly the same as mine, and the checks were all for different amounts of money.  However, many of the people on the 855-382-5097 page said that the letter and attached check was legitimate...which leaves me wondering.  If the letters and checks are legitimate, then why does my letter have a different contact phone number in it than what the people on the other 800notes page have in their letters?

Anyway, after I found the page with the other number about this mystery letter and check, once again I looked for the number in my letter - 855-841-4717 - which led me back to 800notes, where I saw Rene's post above.

Sending fake checks through the mail is a crime so contact the US Postal Service Inspectors:  
https://postalinspectors.uspis.gov/forms/MailFraudComplaint.aspx
https://www.usps.com/

Or take it to your local postmaster..

Looks like mail fraud.  Do not attempt to cash a check from an institution you know nothing about!.  Go to the police, report it.  Go to the post office, give them a copy of all correspondence and ask the postal inspector to look into it.  Do not contact anyone on the correspondence until you have done this.   They may want to get more information out of you.  If it is bogus, don't contact any number on the correspondence at all!

This number was listed in a letter I received, with a check attached to it -- supposedly from Citibank (with whom I do not have an account).  Apparently a lot of people have been receiving these random letters and checks from Citibank -- but the letters have a totally different phone number in them and not this one that was in mine.