Tenant took down smoke alarms. What is my risk?

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Hello hello.  I was just inside one of my units fixing some plumbing, and I noticed they had taken down 3 smoke alarms and the only carbon monoxide alarm.  Only two of the original 5 smoke alarms were still up (though I didn't test them) and they were each inside a bedroom.  They removed the alarms from one bedroom, the hall, and the utility room next to the gas furnace.  

The following is in their lease: 

When one of the tenants came home, I asked her what happened to the smoke alarms.  She said the batteries had all died at the same time and they needed to get new batteries.  I told her that I would have gotten her new batteries if someone had let me know.  (They moved in in March 2015.  I replaced all smoke alarms with new ones from Costco and installed the CO alarm at that time.) 

This is also in their lease: 

I know that at least one of the tenants does smoke, but I've seen her walk down the street to do so.  When I replaced the kitchen drain today, it smelled like cigarette ash had been rinsed down it. But maybe I'm extra paranoid.  

It's 3 women in their late 20's who pay rent on time and who have been very friendly.  We live next door to each other but I'm moving soon.  

I'm worried 1) that everyone will die in a fire and 2) that I'll lose everything I have because someone died in a fire.  There are no sprinklers here.  It's a 1958 duplex.  

What would you do in my situation?  I'm ordering batteries right now, but my concern is that they will continue to remove the alarms (because they might be smoking. But I have no proof.)  But if there is a fire, people and property could burn to the ground.  And then whose fault is it?    

I'm going to email all 3 of them about the smoke alarms (and the furnace filter, because that was completely clogged today when I checked) and quote from the lease, issuing a formal warning, and will replace the alarms with 10 year batteries as soon as they arrive.  Any other suggestions?