We received a notice from the IRS that my husband received $6k in income in 2006 from a job he left in 2005; the notice is a demand for $1600 in additional taxes.

I believe, but cannot get my husband to admit, that he cashed in his 401k, which is a whole different issue than the one that surrounds my question; if he cashed it in, would this have been reported to the IRS as income by the former employer or would all tax notices have come from the company administering the 401k?

I need to track down where this came from quickly as the taxes and penalty are due in 5 days.

The 401K funds and notices would have come from the administrator of the 401K, not the company, unless it was a company funded 401K (which is highly unlikely). I don't know how difficult it will be to get the documentation you need without your husband's assistance, but good luck! If it becomes a really big problem you could file an innocent spouse relief if you feel so inclined.

If he cashed it in, yes that is income. He would have received an additional 1099 from the 401K administrator to list and show on the taxes when you file. At the time he took the money, he would have had the option to pay the 10 percent penalty and also have taxes held out.
Not only will this effect your federal taxes but you will now also owe additional state taxes.

The notice from the IRS only comes after you file and do not include all the income they were given notice of. ** they would have gotten a copy of the 1099 if he cashed this in.

So does he get the mail, so he could have hidden a 1099.

Also if he did not cash it in, he could just call and get a current balance of that account.