ISTA is the dealer software tool they use (ISTA-D diagnostics ISTA-P programming)
I-step is the "version" control for the car, like a status level for all modules. If they just updated the car to latest I-step like they do every day that should just sort it. (press the big "update" button if they can manage that)
Really the selling trader has messed up and sold you a faulty / tampered product. You should be able to return the car as mis-sold / defective.
But.... I had the tamper code after a failed BM3 flash last year. (I found it myself with my own ISTA-D - I never go to BMW)
Worryingly the BM3 code reader couldn't see it but ISTA could. So an unsuspecting BM3 user could think it was OK when BMW would see the tamper code.
The code still stuck even when I flashed back to stock. I had to follow a procedure from PTF to reflash to stock again, then the tamper code just reset and I could clear it from fault memory with ISTA, it's never come back since even with the car mapped.
It's not related to being stock or not stock it's about some checksums etc in the DME software not being correct. You can be stock or mapped and still have tamper code.
Your car could still be mapped for all you know right?
If you update the DME to latest version you will lose OBD tuning ability. Up to you depends what you want.
BMW could just update the car to latest I-step and if a DME update is scheduled it will just overwrite it and put back to stock (as happens with people all the time when their maps are overwitten like this).
Problem is your issue in not on the "normal" list of things for BMW techs so they are stumped...
