Yes this is a concern as the dog should not be producing the hormones that cause milk production.
If she has access to any estrogen type compounds perhaps something you use, creams she licks off of you, birth control pills etc that could trigger this.
It could also indicate she has another hormonal disruption or possibly a tumor. Sometimes you can feel the hard lumps of mammary gland tumors around the nipple areas and those can cause fluid discharge.
it is even possible they missed a bit of ovary all those years ago when she was spayed and she is in a false pregnancy which you can read about here
caused by hormones produced by the ovarian tissue.
I would plan a vet check up for your senior dog.
Hope this helps you!