In theory they could detect it, however I highly doubt anyone in a repair center really cares. Repair centers use Emma (special software) to flash and check devices, and when you relock the bootloader it is impossible to see in Emma if it was unlocked before or not. You can check it yourself easily. If PC Companion and/or Update Service still want to redownload and reinstall your firmware, you will be fine. (Emma does have a section where it shows the DRM status, but for some reason it doesn't mark missing DRM files, I've tried it with a friend's Arc S.) However, unlocking the bootloader is still a risk, and something you should only do if you are experienced and can rescue your device yourself should something go wrong with flashing custom software.
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