Some people can do nothing but lie, even if they know that you know that they are full of shit. It's some weird character flaw, but I don't get it either. Somewhere in their convoluted little head, they are counting on the fact that you're a nice person and won't say anything, and somehow that's a victory...even if you know they're a deceitful poop and they know you know. I couldn't live like that, but lots of folks do.
I agree with Flint. Go in, say "oh there it is" and take it. What's she gonna do? Confronting you would be like calling the cops to rat out the guy who stole your weed.
Good luck Pooka!