Who knows what these are, and what am I doing with them?

[Paul Sandhu]

First closest answer gets one of my 2020 bikini calendars.

[Paul Sandhu]

@Michael Baum @Brendan Curley are both correct.  They are called "sniffers".  They detect Hydrogen Sulfide gas.  The gas is a by product of petroleum refining.  One breath can kill a person.  Even in dilute quantities, one sniff will render a person unable to smell the gas and they can succumb in a minute or two.

All my tenants wear these, and some of them leave them behind in their STR.

So what is my purpose for keeping 3 of them in a basket?