Check out time is at 12a.m.
After that time there is a storage room in the area of lobby where you can leave your luggage as long as you wish.
Check out is 12:00PM mid day and they stick to it unless they are not full. They only provide access to a bathroom and luggage storage if you want to leave later.
If this is helpful please note this. Thank you.
Get an answer from Plaka Hotel staff and past guests.