yeah that's right, they suddenly appears....I see them appears from the ground once, If like they were buried.
I suppose killing a lonely tank is not a challenge according to Valve, they like to trow at you hunters or smokers when you insanely debate your life wasting all your ammo killing him.....and this happens most of the time in the final mision of any chapter, It's very hard to run into the rescuer's vehicle when a thousand of zombies are after you, not to mention you have to dodge the tanks, boomers, hunters and smokers, in single player is very easy to see 2 or 3 survivors wasting their time killing zombies instead of jump into the vehicle, and in the most of the case just one survived.