Susie, you mentioned the hydrangea is now in a pot, which sounds like you have removed it from the problem area? Did you notice any damage to the roots when you removed it? Also if you dug it out of the space, I would have thought you'd have seen a critter there, with or without stripes
Voles are difficult to get rid of if that's what it is. You mentioned also that the pot was well-drained, does that mean you think the problem area is not? I'm wondering why it was necessary to water the plant when it had no leaves, just wondering how were you determining a need?
A picture of the problem area might help, to show what is around it that could be an issue. If you're certain it's an animal then you could try a humane trap, I've occasionally caught voles with peanut butter on a cracker. There are other deterrents but it would really be better to identify the problem first.
