If it's yellow, could have been bile. Maybe she threw up a little.

how old is she? spayed?

my youngest dane was about 3 when she started having spay incontinence. like you, i found big stains. at first, i didn't know if it was drool from one of the dogs grooming herself or if it was pee. i was totally stumped. there was no color, no odor. i didn't figure it out until one day, pea got up from the couch and there was a HUGE fresh puddle right where her back end was.

started her on proin and haven't had a problem since. in fact, i just recently decreased her dosage without a problem.

you need to rule out a UTI first and then go from there.

Yes, she is spayed. And it's not really yellow - very faint stain. Hmm, about the same color as my husband's pillow from drooling :) now that I think about it.
I saw the other post about throwing up bile - that is a problem for Abby, but she has only ever done it once on the bed, and I would KNOW if that was it.

how old is abby? this could very well be spay incontinence.

I bought from Stink Free, the Stink Finder. I call it the pee pee finder. I wait until dark and run a round with the ultra violet light. So cool. It can find blood, drool, poop, pee, vomit and one more.