Thermo is an enhancement for a camera, the camera itself is the sensor function. So you have this:
Sensor array 3 (¥3,000)
|- Camera (Rating 3, 3 capacity)
| |-Thermo (¥500, takes 1 capacity)
| |-Vision Enhance 2 (¥1000, takes 2 capacity)
|- Another sensor function
...
In short, you have a hierarchy that goes like this:
Sensor housing (Drone, vehicle, handheld, wall mounted, armor, most things with capacity)
|-Sensor (array or single)
| |-Sensor Function (Atmosphere, Camera, Microphone, Ultrasound, etc)
| | |-Enhancement (appropriate to sensor, currently only audio and vision for mics and cameras)
A drone should come with a Sensor Array containing 8 functions predefined at the factory. The drone is the housing, the sensor array is the electronics that processes the sensor function inputs. The sensor functions are effectively soldered on to the drone - for example a camera in the front and rear. You can't just pull the camera out and replace it with a microphone anymore than you can do so on your modern day cell phone. The drone is designed and built with specific cutouts and wiring to support the factory installed sensors.
Adding an enhancement to a sensor function (thermo to the camera, for example) increases the availability of an array, meaning that you can't add enhancements after-market (or there are no rules for doing so yet). So to add thermo to that camera would require replacing the entire array. I think that most GMs are allowing the addition of enhancements by paying only the cost of the enhancement (i.e. after market modification of the single sensor), but that is not provided (nor prohibited) by the rules.
It makes sense to me that you could swap one camera for another in a sensor array without having to replace the entire array - assuming that the dimensons are the same. And I don't see any real reason why that simplification can't be made for the sake of simplicity - a rating 3 camera is the same size as another rating 3 camera regardless of the enhancements they have. But swapping one function for another (putting a camera in place of a microphone) can;t be done because the wiring is different, the profile (shape and size) of the sensor itself is different, and the type of data is different which causes the central brain of the array to choke on it.
Also, where have the writers stated that the included arrays in 5E are empty? I've seen GMs allow the players to choose the contents (all the way down to Enhancements), but I've not seen that officially (or semi-officially) addressed by anyone associated with Catalyst.
So, again, there is no price given for adding an atmospheric sensor to an array. Or cyberware scanner, or geiger counter, etc. In order to add an enhancement (thermo, low-light, audio of vision enhancement) you must first have the appropriate sensor function to add it to. So the blank slate model doesn't work unless the sensor functions are free to add and change as the player desires. Which I have seen some people play it that way, though now Rigger 5 makes quite a deal about how integrated sensors are and thus how difficult they are to change.