It's back again, in another pot. Very hairy stem, 3-5 lobes on the leaves, think the flower was lavender-colored.
Ivy-leaved morning glory, Ipomoea hederacea?
I think that's the closest match, but it doesn't seem exact.. The other thing is that the flower never really blooms, but it does produce seeds -- I saved them from last year, but never planted them.
If it makes seed it must flower. Maybe you could observe carefully and try and get a photo. Or maybe show us a picture of the seed if you still have it.
Here are the remnants of the bud and seeds from last year. I only found it in one other place on the web, which I've linked. The poster includes a picture of the "bloom," and describes perfectly what happens to it. Maybe the bloom is just green? The other poster never found out what it was.
Here is a link that might be useful: Another example elsewhere on the web