is the product always on?

I just bought one. The answer below is wrong. It is strickly a gas detector. It has to be turned on. It is not always on. It takes 15 seconds to warm up and then the green light stays on. When gas is detected either the yellow or yellow and red will light depending on the severity of leak. However, good luck! I also just retruned it as it was detecting gas literally everywhere, indoors, outdoors, no matter where. Treated very rudely when I returned it. Wanted to get another but the clerk didn't know where they were and after asking said they didn't have them. Got home and found they have many of them.

It has a pocket-clip that is also a momentary-contact switch. When you press it, the detector comes on. When you let go of it, the detector turns off. This is an A/C voltage detector only. If you need to detect DC voltage or gas, you need something else.