Welcome to the forum! Seems this '07 rust problem is suddenly getting a lot of attention.
In answer to your question, rust grows. As anyone knows, every car is bright, shiney and solid when it leaves the factory. After awhile things happen.
As for your case, small holes can be missed, especially if someone isn't trained to look for that particular spot, or defects can be intentionally overlooked. A 2" hole is pretty big, but none of us can go back 2 years in time to find out what it looked like then. Two years is a pretty long time to go back to somebody and accuse them of avoiding a defect. I know what they'll say, "Our service department would NEVER do that." So I can save you asking.
So yeah, to give them the benefit of the doubt, they probably weren't expecting that problem and probably didn't look real hard for it.
^This, did the sub-frame have rust two years ago? probably. Did the dealer know about it? maybe, maybe-not.
Boxed frames/sub-frames (like the ones on the patriot) tend to rust from the inside out (salt, etc. gets trapped inside if the underside is not washed thoroughly and causes corrosion), so while it may have been rusting on the inside and possibly had surface rust visible on the outside there is a good chance it didn't have any visible holes yet (or just small "pin" holes that could easily be missed as Ignatz mentioned) two years ago and the large holes it has now may have occurred since.
That being said as a general rule, living in the salt-belt (PA) I never buy a used vehicle without looking underneath myself, even if a dealer isn't trying to scam you their service departments are the end of the day are being paid by the hour and generally want to inspect the vehicles as quickly as possible (especially since this isn't something the dealer is getting paid to do, except from the eventual sale) and may miss things that aren't obvious.
My XJ Cherokee passed emissions inspection two years in a row with a cracked exhaust manifold (common issue on 4.0L I6s, I knew it was cracked, but the crack was small and only leaked when cold, so I wasn't going to fix it unless I failed inspection) before one finally caught it and told me I had to replace it to pass, and these were shops I was paying to do an inspection on and had no incentive to pass a vehicle that shouldn't have (if anything they had incentive to fail it in hopes of getting the repair revenue which would have been much higher than the inspection fee).