Long story short...I didn't have Vodka so had to use distilled water in the airlock. Life got in the way and over a week later I finally got back to the beer today. Fermentation has completed but the airlock contains a mixture of stagnant blowoff. A quick inspection and sniff test makes me think it's not contaminated, but the airlock is definitely sour-smelling from the stagnant blowoff. I've never not used Vodka or Star San and am wondering what the likelihood of a contaminated batch would be from an otherwise undisturbed airlock.
I think the likelihood of a batch getting contaminated this way is pretty low. Certainly be careful with cleaning and replacing the airlock. But generally speaking the airlock serves as a blocker for air exchange. If the airlock did become contaminated, the contaminants would still need to work their through the airlock and into the fermenter, which isn't super likely unless there was a large temperature or pressure difference. And even it if did there's a decent chance it wouldn't survive in the quantities that would get through.
To be on the safe side, I would check the gravity, clean the airlock, and give it a little longer. If when you're ready to package it there haven't been any large changes in appearance, gravity, or smell, you're probably good to go.