Smoke detector beeping even without batteries! Help!

Our smoke detector started randomly beeping last night about midnight so I immediately changed the battery, but it's still beeping! I'm pretty certain it's not hooked up to mains power or the security system.

I took out the battery to clean it and it beeped even without the bloody battery! I gave it a dust but couldn't get all the dust off.

Is it just old? I tried taking the battery out and pressing the test button to reset it (as per googles instructions) but nothing happened.

It's driving me insane and I worry about it not working.

I'd replace it. They're not very expensive or difficult to install.

How old is it? They have an expiry date just like batteries.

Are you sure it isn't hardwired? How old is your house?

Our old one beeped even without batteries...I always thought it was a safety thing that it did that to remind you there were no batteries...?!

Def get a new one.

Thanks everyone. House is old (1890s) and renovated most recently late-90s. We moved in here in 2009/10. I replace the battery regularly, but I think the unit is prob old.

Grrrr to the beeping. I love smoke detectors though, so off to the hardware shop I go!