This is the material. I wonder if I can put an electrical heater inside or if it melts or catches fire. The material is called 600d mylar.
They are probably very old. We had a problem like this at our previous house. It would go off any time of the day or night for no reason at all and the only way to shut it off was to tear it off the ceiling, wait for a while then hook it back up again. When we got tired of this and phoned the fire department to find out what gives, we were told this is a VERY COMMON problem with old detectors (ours was 20 yrs old). As they get older they get hyper-sensitive. Long story short, there's nothing wrong, it's just old. Pitch it and get a new one.
If you're talking about the parts that stick off the hood, and connect it to the rest of the car, I think that is part of the hood. So you'd need a whole hood. Maybe I don't get what you are describing though. If you could take a picture and post it somewhere, that would probably help.
If it is hard-wired with battery back-up, it may be an alarm because there is no battery. If it's battery-only it should stop. If it is the wired type, you can remove it. Just turn it counter-clockwise about an inch, it should unhook from the base. Now squeeze the sides of the little wiring connector and wiggle it out to unplug it. Now you can set it where you can get some sleep till tomorrow. But PLEASE replace it.smoke detectors (although sometimes problematic) do save lives.
