I got a certified letter from the IRS?

My husband got a notice that the mail lady tried to deliver a certifed letter from the IRS (well,actually the line marked Large envelope is marked) and we have to pick it up Monday. It is going to drive me nuts over the weekend, so I was wondering if anyone may have any ideas as to what could be in there? I... show more My husband got a notice that the mail lady tried to deliver a certifed letter from the IRS (well,actually the line marked Large envelope is marked) and we have to pick it up Monday. It is going to drive me nuts over the weekend, so I was wondering if anyone may have any ideas as to what could be in there? I already filed my taxes in February and got the refund as well. Could it be an audit, even though they already sent the refund? I have no idea what we would be getting audited over as my husband works only one job and we have no other income.
Update: I just wanted to add that is the first letter we have ever gotten from them. There have been no others that were ignored, that is why this is so odd to me.
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