So not found for over several weeks? Dehydration I can see in only a few days, maybe a week. Think of the rule of 3’s:
1. 3 minutes without air.
2. 3 hours from exposure.
3. 3 days without water.
4. 3 weeks without food.
Yes, dehydration is quicker and may be the cause. On the other hand, if the dog was older and had health issues, the lack of food (or perhaps medication) might have had an effect.