I wanted to hotbox my shower a few weeks ago so I did some research on the issue. I believe that cannabis smoke is somehow too thin or something, so it doesn't get detected by your alarm.

But the I realized I didn't even have smoke detectors, but automatic sprinklers, and those are set off by heat as opposed to smoke. So I went ahead and smoked a fat spliff as I showered... no, it didn't get soaked.