Gosh, I'd write a note and say, "did you take our Tundra? If you promise to love her, we'll be okay with you taking her. We just want her to be happy, and maybe she will be happier with you ..." (and explain the situation a little.) That's so good that you have a good attitude about it, and do want her to be happy, and maybe if you leave that note you'll get a response and maybe you can visit her sometime!

If I "stole" someone's pet, I'd avoid that person at all costs, and assume they hate me, so letting them know you don't is okay!