Friend of mine who was vacationing across Europe (from Barcelona to Vienna) said that twice he had a case that in the middle of night someone opened his hotel's door.

In one hotel room had a chain, but in second times had not, friend woke up and that someone who opened doo said 'sorry', closed the door and left. Friend moved the table against the door just to make sure that won't be any other surprises. Next morning asked the reception 'what the hell' but they said that no one from their personnel opened the door.

Once friend was back in states he ordered this device from Amazon and now carries it with all his travels and tells me that I should do the same.

I personally never had such problem in past. Have you?