Howdy,
I don’t think it is anything to worry about,
I have a couple of thoughts,
It looks most likely to be within the vagina or immediately adjacent to it,
A 59 year old is unlikely to have a tampon in situ,
So I think it is a little gas within the vagina is a possibility.
It is possible that it is posterior to the vagina and on the transverse view I am in 2 minds about if it is within the vagina or the rectum,
Either way one possibility is some normal gas.
Another possibility is that it is some tape that has been inserted as treatment for prolapse.
Looking at your images again I think this is another strong possibility as the tape is inserted adjacent to the urethra and so the location is about right and this would explain why it caught your eye so much and also why it looks short on your sagittal images and lengthens out in your transverse image.
Had she had any prior prolapse surgery?
Steve.