A tenant keeps forgetting to turn off a gas stove burner and fire hazard

[Meri Hamada]

I have 3 individuals renting a condo together that is 3 bed 3 bath. Everyone has their own room and a bathroom and they share common space such as living room, dining room and a kitchen and they are one the same lease. One of them reported to me that a gas stove burner was left on overnight. I’m aware of this happening once previously last summer, but thought it was a one time accident. Apparently this has happened 4 times in the past with the most recent 2 incidences happening last month.  I sent email notification to all tenants the following day saying that this is unacceptable behavior since it can potentially cause a fire and carbon monoxide poisoning. 

 I posted a sign to turn off gas as a reminder in the kitchen. I also bought a fire extinguisher and placed it in the kitchen. 

Since there are 3 people living in this condo sharing the kitchen, I am not able to pin point exactly who has been doing this even though I have some idea.  I spoke with the one I suspected forgetting to turn off the stove but the person did not confirm.

I told them that if this repeated incident does not stop, I need to investigate further and take action accordingly. 

What else can I do? Do you have any advise? suggestions?