How did it get there?

Now here's a bit of a mystery.
A couple of days ago while admiring my herbaceous border, I was stopped in my tracks by a bright yellow flower which I'd never seen before.
After some investigation, I've identified it as Iris xiphium 'Lusitanica', one of the bulb Irises. Very pretty it is too and apparently well suited to the conditions (dry, sandy, sunny).
Thing is, how on earth did it get there?
I certainly didn't plant it, I would have noticed it had it been there in previous years (which it wasn't), so where did it come from?
A couple of possible explanations; Squirrels frequently dig holes and bury things in my garden during Autumn and Winter, (springtime usually brings a good crop of Hazel and Walnut seedlings brought in from neighbours' gardens), so could it have been dug up and replanted in my garden by a Squirrel?
Or could it have grown from seed? Do bulb Irises even produce seeds?
Must admit I'm baffled by this one.
Anyone out there have any other ideas?
Also, I'd quite like to keep it going and have a few more, so what's the best way to propagate some more and increase my stock?

 

How strange. My mate has been doing his garden lately, after taking an interest for the first time ever. A few days ago he was whinging about his lack of plants. Then the next door I called round and spotted a lovely flowering plant, it looked quite mature. I asked where he'd got it from because I know for a fact it wasn't there a couple of days beforehand. He looked and was shocked. He hadn't put it there, and there's no way it had had time to grow from seed to 2 ft tall with flowers in a couple of days.

 

It's Mr. Nobody interfering again!

 

Nice One JWK.

Ah, well whatever the explanation, it's one of the joys of gardening isn't it, when nature throws in these little surprises every now and then.