I'm with Splatcher on this. I rarely feel hungry or thirsty even when running for several hours. Despite that I've had one or two bad experiences when I've become pretty dehydrated on a run and definitely feel stronger in the last miles if I do drink & eat. Having said that, I don't eat or drink anything like as much as I see recommended sometimes and nothing "special". Very roughly speaking I only take a drink if I'm planning to run for more than an hour in the tropics (longer in the UK depending on the weather) and drink one or two mouthfuls (100ml?) each hour or so, and it's usually just water or diluted orange juice. I only eat if I'm out for more than about three hours - then it's a banana or handful of raisins, slice of malt loaf.
And I don't think we're unusual in not noticing thirst even when becoming dehydrated. I have lots of friends who do manual work all day under a tropical sun but rarely drink during the day because they say they are not thirsty. They do, however, get frequent headaches caused by dehydration. I know it's dehydration because when they come asking for paracetamol I make them drink a couple of glasses of water and only give the tablets if they still have the headache 20 mins later. They rarely do.
Having said all that, I'm not sure just drinking a lot after the end of a run is necessarily a sign that you were too dehydrated. As Mike's excerpt suggests, being a little dehydrated may actually be helpful but I'm guessing it's a fine line between good dehydration (= less weight) and bad dehydration that impacts your performance negatively. Even though I drink a little while running I'm usually thirsty afterwards (I can easily down a couple of pints of water, milk, tea &/or homemade ORF in the hour or two after a 30 miler). I don't see that as a problem as long as I wasn't becoming wobbly legged or simply finding the going unusually tough in the last miles. Feeling thirsty might be an adequate indicator to avoid this for some people, but not for me.