If it never smelled like that before and you haven’t cleaned it with any “special” cleaners etc, the odour may be coming from a leaking battery.
Do not charge the phone or try to turn it on just to play it safe, before you have opened it and can check what is going on.
Here’s the ifixit Xiaomi Mi Note Battery Replacement guide, that may help with opening it.
If you can’t get it open go to a reputable, professional mobile phone service and ask for a quote to check the phone. Do it sooner than later because if it is a leaking battery, the acid can really cause some damage to the phone, if it hasn’t already done so.
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