It does sound like they use the word engineer to help generate more money.  If the rental property has a gas boiler the carbon monoxise alarm also falls under the perview of the Plumber/Heating engineer.

As a side note testing a Heat/Smoke/CO is not just pressing the button, the person should also check the expiry date of the unit and locations (eg Aico states I think, smoke alarms to be at least 300m from a wall or or any obstructions).  It would be perfectably acceptable for you to ask that engineer to make a note of the expiry date. 

A CO detector would normally be mounted on a wall somewhere near the appliance.  CO is pretty much the same density as N2 or O2, so it doesn't rise.

This thread prompted me to check my CO detectors.  One of them rather unhelpfully gives the expiry as 7 years after installation.  If I hadn't written the installation date on the CO detector, I would have no idea when it expired.

I normally write on the inside of smokes the install date with a Sharpie (other brands are available) but I image that many people do not.  Likewise I bet many mains powered smokes/heat alarms dont't have the install date or have they used FireMate to seal the building fabric penetrations. 

I do that (write the installation year and month) on almost anything, and would encourage those who don't, to do so.  Even the living room clock has a long list of new battery dates on the back of it on a now rather yellowing paper label. Even in a situation where you are the person who did it it is quite hard to remember dates accurately (now thinking things like was it before or after lockdown, who was PM or what age the kids were only gets you so far, or for some of my older stuff, what century it was... ) Once you get into 'installer unknown' territory it can be very hard indeed to work out how old things are. Luckily smoke alarms tend to fail too sensitive and false alarm when superannuated, I'm less sure about CO.

Mike.

I do that (write the installation year and month) on almost anything, and would encourage those who don't, to do so.  Even the living room clock has a long list of new battery dates on the back of it on a now rather yellowing paper label. Even in a situation where you are the person who did it it is quite hard to remember dates accurately (now thinking things like was it before or after lockdown, who was PM or what age the kids were only gets you so far, or for some of my older stuff, what century it was... ) Once you get into 'installer unknown' territory it can be very hard indeed to work out how old things are. Luckily smoke alarms tend to fail too sensitive and false alarm when superannuated, I'm less sure about CO.

Mike.