What's with all the barking all of a sudden?

[Dumpylump]

My 6 year old cairn terrier has taken to barking. A lot. The minute he goes in the garden he's barking his head off, so I have to bring him in again since I don't want the neighbours to hate us. He doesn't do it when we're out for a walk though.
He is in the kitchen now, as he is every night, and has been barking non stop for nearly half an hour. I have been downstairs twice, he doesn't seem ill or upset......it's driving me insane

Thanks Oldbag I'll give it a try. He barked for ages last night, and then eventually stopped. But he woke me up in the middle of the night doing it again....I went downstairs and he wanted out, so I let him out and he stood just outside the back door barking like mad - at nothing that I could make out. He wouldn't come in, until I bribed him with a wee bit of chicken (which really went against the grain, but I was terrified he was going to wake all the neighbours).
I've never seen, or smelled, a fox round here, but I don't suppose that means there aren't any!