Long constant beeping sound after locking the car - Nissan Micra URBIS (K12) 2005


I bought Nissan Micra Urbis MK3 FL (K12) 2005 yesterday and have a beeping problem after locking the car. The sound lasts for about a minute, two and is constant. I checked if everything is properly locked (windows, door, boot, bonnet etc ) – it is. The radio and ignition is off. The spare key is not left inside (I separated the keys as well) and not near the car. I keep the distance from the handles. I’ve waited 5 sec after turning ignition off, then I locked the car – keeps beeping. I tried 10 and 15 sec after – the same. The battery in key is changed.
Any ideas?

btw I am new here, so Hi everyone! ;)