Dibby

They could be a plug-in chip that fools the ECU into dumping a load more fuel in than it should. If it's an El-cheapo one from ebay, get rid of it as quick as you can, these things kill engines. Look for something that looks a bit straggly or a box with wires coming from it, maybe wrapped in a plastic bag to keep it dry, covered in zip-ties or insulation tape? Your garage will be able to sopt this easily and unplug it for you.

Doubtful it will be a replacement ECU when remapping will be cheaper and easier. There's a zillion and one revisions of ECUs and it will have to be the exact right one for the car, too much hassle to replace the ECU really.

If there's no extra chips then it will be remapped, this can be played about with by any of the tuning companies, garages won't be able to help you here.

Your garage will know what's going on when it goes back there and will be able to tell you if it's a remap or a chip.