Unusual plant ID

We're trying to ID this plant in a client's landscape. Plant has multiple thin stems and acts like it wants to climb and almost seems like it could be a weed or invasive. Any help appreciated.

 

My first thought was Buckthorn (Rhamnus)...but---? Leaves do not appear toothy.
Ridding my own garden/yard of this vigorous invader is an ongoing battle.

 

Could it possibly be a young Lonicera japonica-Japanese honeysuckle?

 

I can't tell what it is, but it is not a Honeysuckle.

 

A shrubby dogwood, like Cornus drummondii?They can form suckers and colonize a yard (maybe the original individual is now gone). I see some photos that look just like yours, and others that look nothing like it. It's not a plant that I know - I just thought the leaves looked dogwood-like and are opposite.

 

Thanks, wcutler--I'll check tomorrow to see if the "elastic" threads at the veins hold the leaf together when I gently pull it apart... My original thought was that the leaves look like a dogwood, but the stems seem so delicate, almost vine-like.

 

Yup--looks like it's a dogwood. The client said they moved in to an old house with old plantings, and chances are that this is the last hurrah of a dogwood that had been planted there at one time in the past. Thanks for your input, everyone!