Gina,
I total sympathize with what you are going thru. Sango is doing the same to me. She just turned one on March 24th (she's a few days older than Lexi). This past week she has been driving me insane. At least Lexi is good and doesn't pee while you're not home. Sango pees when I'm not home AND overnight where she's not supposed to. I'm in the same boat as you. I never catch her so I can't reprimand her for it.
But I think it's back to the drawing board for me. This morning before I left for work. I left her in the X-pen that is lined with weewee pads. And on top of that I crate her overnight.
Wish you luck with Lexi.
You're not alone in this girlie!