Gee everyone, thanks for all the terrific replies, what a great group of people. Yes, I have taken Mookie in to our vets for a check-up on her discharge. They just weren't too concerned (this was during the first year of her adoption), so I took her to another vet (because I was a worry wart) who kept her five days to "run tests". When I picked her up the vet was too busy to speak to me and the vet techs just insisted that nothing abnormal came up.
So, basically, I just stopped worrying. FoxyFantastic, you asked how I managed to put up with it for five years. Well, I didn't... because it happens mostly at night. Both cats have their own bedroom on the first floor (husband doesn't like cats in bed with us) so at night in they go. Just four months ago we put them in a new kitty bedroom, next door to ours. So that's how I now know that the yodeling I heard on my nocturnal wanderings in the house over the past five years were a constant and I'm guessing nightly occurrance (I had chalked it up to the fact that she had heard me walking around and was meowing to get out too).
But, now that I am armed with all these wonderful suggestions about estrus, thyroid etc I am making an appointment with my new vet. Thanks for all the help, I'll post back again letting you know the outcome.
Oh, funny thing. About two years ago I took Mookie in for her rabies vaccine and the vet on call that day was fresh out of school. As she did a health check and palpated the cat's abdomen she kept asking me "are you sure she's spayed?". To which I replied, "well, I assume so, as that's what I was told".
Anyway, its all adding up now isn't it?