Oh David, I am so sorry to hear that your precious Nikki has gone missing, but please do not give up hope of finding her or of her coming back home. It's been what now, 3-4 days. We had a cat that went missing and after weeks I had given up hope because they had our laneway dug up to put in new piping. They were big cement like tunnels and we figured that he had gotten himself lost in them. BUT, after 48 days of being gone, my hubby heard this meow outback and sure enough, our Mittens was back home. Very skinny, full of fleas, but finally back home.
So please keep the faith.

Have you put one of your shirts outside for her to get a scent of it? One that you have just worn and not washed yet. Put flyers up or gone around the neighbourhood with a picture to show everyone? Talked with the neighbourhood kids to be on the lookout for her?
If I have repeated what already has been mentioned, I'm sorry, but I didn't read all the posts or threads made in regards to this.

Prayers and positive thoughts on the way, that Nikki will be back with you very soon, safe and sound.