If you earned below the foreign income exclusion amount you won't need to pay anything. I believe you need to spend 10 or 11 months out of country to qualify, so i doubt the 2009 year will be legit. For filing late, you do accrue some pretty nasty penalties, but this only applies if you earn above the exclusion.The amount is up to $80-90k a year, and yes you still technically have to file, no matter if you owe taxes or not.
I'm asking if there is any practical consequence to not filing, ie how would the IRS know about my overseas income?
The IRS can find out about the income in several ways. Firstly, you are required by law to declare your foreign bank accounts in the income exclusion form. So if you have filed previously, then they are at least aware of the bank and the account you were using at the time. I have heard of banks sharing information with the IRS, but i dont know exactly how or why the IRS would ask, or when/how the bank would supply that information.
honestly, if you are earning below the exclusion it is MUCH better to just file and forget in my opinion. If you are earning above the exclusion amount, you are taking a huge risk by not filing. If you ever work in the US again and file, you will need to fill in those missing years, and then either have to lie or pay a huge fine. SO I would recommend just filing it.
Last - if you have a bunch of years to file together, you should be able to negotiate a discount with your account to do them all at once.