Another Intake Gasket Question

I just purchased a 2000 Malibu V6 in mid May and I haven't had any problems with it, but I do have a question. If my intake manifold was leaking, how long would it take for the Dex-cool residue to dry up and go away off the side of my heads. I ask because when I went to purchase the vehicle we took it to my mechanic to have him check it out. He came back and said that the intake manifold gasket was leaking. So, before we purchased it from the dealer we made the dealer replace the leaking gasket. The dealer then told us a couple days later that when there mechanic took off the manifold, the gasket had already been replaced. They also said that if it ever was leaking to bring it back in and they would change the gasket though because it was a term of sale. Our dealer was a large dealer in northeast indiana, Kelly Automotive. I just took a look yesterday and saw some black gummy residue where the manifold used to leak at. Does that mean it is still leaking or could it just be left over from when it was leaking however long ago. I tried to clean it so I could watch it to see if any more leaks out. It was really thick and greasy on the rag. I forgot to bring my camera to work, I'll get some pictures up after lunch if it would help.

http://images1.snapfish.com/347828%3A96%7Ffp347%3Enu%3D328%3B%3E696%3E984%3EWS NRCG%3D3233%3A3%3A%3B749%3A5nu0mrj
Here is what I was talking about