To further discuss the OPs problem we will have to wait for feedback relating to the effect moving the smoke detector has had.
You will notice that the OP has mentioned 3 years without problems.
I could say that about a few thousand other people too.
Get one and test it in a domestic environment. Your concerns while valid do not translate into a reason not to consider the equipment in the vast majority of cases.
I am not going to deny an alarm system to someone if their budget is limited and their choices are limited.
At the end of the day NO alarm whatever the spec or design will prevent a break in. Only physical security can do that.
However its existence can and does deter burglars.
If I fit a Yale alarm I know it has been fitted and tested to a higher standard than a homeowner would do so at their first attempt. I know the devices are fixed properly not stuck but screwed. I know the siren is out of harms way not where it can be reached. I know any features available have been utilised to best effect and that the system works as designed and all signals are reaching the panel as proven by walk tests and a 'trial break in' scenario.
I know the phone works and phones out and the filters work and the numbers are correctly set and the in code is as needed.
The owner knows if the alarm goes off they will get a phone call. that's reassuring as a monitored alarm to an ARC is no guarantee that the Police will attend or even that keyholders are actually within 20 minutes of the property.
How many owners of monitored systems organise holiday cover for their keyholders or even keep the contact details current?
I wasn't 'told' about the possibility of setting alarms with a door open. It was I who originally posted that information.
Of course you are right loads of people forget to see if their back door is shut when they leave the home dont they?
But even if they did the movement sensor would get them anyway. I always do an alarm plan assuming a door can be bypassed by breaking through (ie kicking out panels) so I always cover the room with an entry with an additional PIR.
But then I have experience of looking for risks and doing risk assessments even to the degree that if a risk is great I actually suggest that a Yale alarm isn't the right solution and that the homeowner should approach another local provider for a graded system. (Yes i do that).