If they have not placed that sticker under the hood, then you need to go back to the dealership and speak to someone, usually the Service Manager if available or their assistant, and let him or her know that your PCM (powertrain control module, ECU, etc. it all means the same thing, your powertrain computer) was reflashed but no sticker with the new calibration code was placed under the hood, and you need that information (take your service paperwork from where they reflashed the PCM with you just in case). Don't worry if they ask you why you need that info, that is really none of their business; their only concern in any of this is to just do their job correctly. The sticker identifying the new calibration code is required by emissions laws, as it is a change that affects emissions status, and so they have to comply with the requirements when an automaker makes a change like that, and you do not have to explain why you want it. It's simply a matter of making sure they did their job right, so that no future dealership will charge you to scan the ECU to identify that calibration code should you have another problem in the future out of warranty and the next dealership needs to know which calibration you're using.