The Leeo is an unusual device, so it’s easy to get confused about what it actually does. The Leeo doesn’t call their device a relay, and it’s not in that device class. It’s an acoustic sensor.
In the US, the sound made by a smoke sensor in a private residence is set by the US standard. That is all smoke sensors make the same sound. So the Leeo and other similar devices just listen for that specific sound pattern and trigger off of that. That’s why they can work with any smoke sensor and they don’t have to be wired in to the same circuit. (Hence not a relay.)
If you want it to use IFTTT to trigger a report in SmartThings as a smoke sensor, I believe you’d have to use the capability.smokedetector
But you’d be dealing with potential lag of about 15 minutes and depending on your Internet, your power, your SmartThings hub, and the SmartThings cloud to all be working while there was a fire in the house.
It might still have value for some use cases. I’m not sure exactly how that particular capability works, hopefully one of the expert coders will answer.
I’m assuming you want SHM to do something based on its reporting as a smoke sensor, as otherwise you could just go ahead and turn on individual sirens, lights, etc. through IFTTT if you wanted without needing a virtual device.
You can also just have Leeo send a notification directly to your phone, which is what it’s intended for.
You probably already know this, but for others who might be reading you can have the same event used as the “if” for multiple IFTTT recipes. So it’s the equivalent of an “and” even though IFTTT doesn’t have ands in its recipes. So you could have the Leeo IF trigger your siren, your Hue lights, other smartthings – controlled lights, a notification to you, and your racchio sprinkler system all at the same time. It’s just that you have to set up one recipe for each event.