Some bugs I found with a quick test:
-does not disconnect from manually moved trains
-does not attach to trains when placed on very front or back in some orientations
-does not reattach to another valid assembler/furnace when removing currently attached entity

Please let me know how to reproduce

Manually moving trains are taken out...
So I am afraid you have to show me what's wrong.

I think you are trying this on old save?

I have test played this mod, and didn't found this problems

also you didn't respond on the fact that this mod is 10 times as small as yours
and will work on every factorio version(in opposite off yours)

And the event driven aproach?

About the Re-attaching situation: Will be fixed next update

Thanks for pointing that out missed that situation.

About the train positioning, to be honest was meant to be.

At least if you meant that sensor need to be close to the middle off the carriage?

let a train automatically drive in a station, plant the sensor, set it to manual and drive it out: sensor will keep showing the removed trains inventory

the changes you made to find_entities counterintuitively make sensors not detect entities where a smaller bounding box would

where else? Do you start a fresh game just to test a mod?

I don't care for anything before 0.13. There is a much more versatile sensor in Smarter-Circuitry for 0.12.

The event driven approach is a nice idea, but needs a lot more checks to make sure edge cases don't results in sensors keep reporting disconnected entities or failing to reconnect with new entities.
There was no noticeable update time improvement over polling every 200 ticks.

I have no clue what you mean by smaller. Your version already has 100 lines more than mine, while requiring even more validity checks.

Edit:
Why would you on purpose limit the sensor near trains to the middle instead of using the whole 6 fields per side?

Edit2:
feel free to further discuss this on the forum or via pm, as the mod portal has no notifications.