I accidentally drove in 2 in an automatic car for about 15 minutes at ~30-40 mph, and about 15 minutes at about 50 mph. As expected, the gas level decreased dramatically. However, after a day later of driving in D, I noticed the gas level kept dropping more quickly than usual. What could be the problem?
Driving in 2, or second gear, at the speeds you noted won’t cause any problems. Just drive on and refill your tank when you need gas. The guage reading is likely due to not being on level ground or some other not important issue. Your car is fine.
I don’t think you have any damage. Driving that way will use more fuel and unless you are careful about measuring mileage it is not easy to tell if there has been any real decrease in mileage and we all tend to see what we expect so the reduced mileage is likely not real. How are your measuring the mileage? Generally it takes at least three fill-ups while keeping proper records before you know what your mileage really has been. Looking at a gauge is just not accurate enough.
If you ALWAYS did this, you could significantly reduce engine life. Your short time will have no effect.