Cats can 'mark' with their scent gland rgeardless of their reproductive status.
Marking can occur for a variety of reasons not the least being territorial or anxiety with a new arrival, human or pet.
My female, Ginger, used to just shake her tail, nothing coming out, whenever she would get excited. Now that she's lived at our new place and seen Sammy do his front paw stand and spray routine, she's now squirting outside on plants from time to time.
You may need to get a spray bottle and keep it handy. Give her a blast when she's getting in position, and re-direct that thought.
Oh, and some Odor Neutralizer will help avoid re-spraying a certain area.