If there is a sensor with a wallet and someone steals the sensor. Wouldn't it be easy for him to hack the sensor and find the seed?
The software and network surroundings of these sensors will have fewer and fewer bugs over time because every attack fixes a few. Eventually, they will become secure enough to not be hacked again.
If the codebase of the sensor is very small, one could achieve mathematical unhackability. But this gets exponentially harder as the lines of code increase.
Even if you have mathematical unhackable software, it would still be possible to hack it, if you know where the sensor is and have access to it.
There are still some countermeasures you could take to make it harder for the attacker to get your funds. Similar to what hardware wallets are doing already.
But you won't have physical access to a lot of sensors because they are either protected in a safe space or at a secret location.
Even if one were to get access to and hack a sensor, the removal of the sensor and/or damage to the sensor would probably be a bigger loss than the stolen funds because sensors only need small amounts of IOTAs at a time.
Furthermore, one could implement a detection mechanism that notices once the sensor's case gets opened or damaged. Upon such an event, the memory of the sensor could be wiped.