IR sensors are sensitive to heat changes within the chip. That includes body temperature, but also a passing cloud within your property, etc. Using dual tech sensors, means it will only trigger when both IR plus microwave are detected, reducing false alarms. Other technologies includes ultrasonic, etc. I normally use Optex brand
I agree, I would use a proper outdoor sensor. I've got three PIRs to trigger the outside lights, two under a canopy and one near the front door, each makes an individual sound inside the house.
I've found too that cheap PIRs can be unreliable in windy weather, producing false triggers. My most reliable PIR is a Voltek 1816. It's not dual IR and microwave but relies on re-triggers in a short time frame. You can select 1 to 4 pulses within a 5 to 30 second time frame, and you would likely to need a control box which is an extra cost.
The Voltek website isn't https which is poor for a company selling security products! Be aware
Indoor IR sensors only, are designed for controlled environment, in which you may close a window or shade to reduce light changes, or eliminate the air flow within a room, that may move a curtain an trigger the sensor, etc.
For those who are up for a little DIY, I've been playing around with the RCWL-0516 microwave sensors and they work great... super cheap too. If you aren't averse to an arduino type project, then these sensors are an excellent option for sensing motion in a 'challenging environment' (tree shadows, sun exposure, higher temperature areas, etc...).
Super affordable... I got 5 shipped for $9
10-15m useful range (can be reduced by adding a resistor)
works great even on hot summer days, unlike PIR that have reduced range when the surrounding surfaces approach human body temperature
not affected by tree shadows
DIY soldering/programming involved
Sensitive to wifi without a dc/dc converter (use an 11VDC step up if using with esp8266)
Not good for battery use (idles in the mA range)
Pro/Con depends on the application:
can see through thin walls, doors, windows
omni-directional can sense in a 360degree sphere (there are similar DIY sensors that can do 180* for a bit more $$$)
can also sense animals (uses 'doppler', so the size and speed of the 'moving bag of water' matter)
I guess most on this site might be looking for more of a pnp device like the bosch combo sensors, but just wanted to put this out there for those who for example, might be having issues with installing an am312 to cover the front door and side gate. Those last 2 pros are huge!