Why did my smoke alarm just go off? No smoke, no pans left on, no flames rising from the computer. No fire outside in the garden. There must be a reason. I randomly asked Google the question ‘can spiders set smoke alarms off?’, I got the answer above. We tried blowing and detritus out of it, but when hubby put it back together it did it again. Why? I don’t know, but we will be buying a new smoke alarm tomorrow.