My mil reacted like this when she was caught sneaking alcohol. I kind of think an addict in denial lashes out when they are challenged. .
My mil would get drunk most evenings, every since childhood she'd drink most evenings, but I also had very strong suspicions she was drinking during the day, just small changes in her behaviour, and I could smell alcohol on her when she'd be picking up grandchildren at the school, the extended family were very dismissive of me when I mentioned it but one day when she "popped in" my home, while I was at the toilet I heard her move into the kitchen and when I came downstairs some miniature bottles of gin I'd been gifted for a bday a few month before were on my counter and three of them had gone, I asked her if she'd moved anything and she said she'd taken the gin because she knew I didn't drink and my bday was months ago (true) and she suggested I was being petty by noticing they'd gone. She even suggested I had an alcohol problem because I noticed three tiny bottles missing 
Because you hadn't seen her drink it, or you haven't seen the empty bottle in the bin or anything, it sounds like she's drank it in secret, I'd maybe be prepared for her drinking to be a lot worse than you think, mil used to have dozens and dozens of bottles under her bed, and when she knew family were aware of how bad her addiction was but she refused to admit, she started putting her empties in other peoples bins. One of her neighbours approached dh because mil was putting a bin bag full of empties into her bin every week.
I'd not give her the rest of the wine, I'm not saying don't give her a gift at all (all though I wouldn't want to) but if you do, I'd remove the wine and buy some flowers instead and if you were planning on taking her to a pub for a meal I'd seriously rethink that. I hope I'm totally wrong but if she's on the same pathway as my mil was, you're in for a very hard time and I'd not be sharing any alcohol with her any time soon.
Best of luck.