I heard of many things. There is an infinite repertoire of possible techniques how they could have verified that nobody entered their room. Ranging from the low tech (dust, crisps, hair stuck to the door frame, filling the room with angry bobcats), through the social engineering (befriended the head of security who told them), to ultra high tech (lidar triggered camera array reporting to a satellite ).

On a balance of probabilities without hearing more on how they specifically verified it I will still assume the alternate hypothesis. Which is that someone on the internet is very sure about something and they are wrong.

It is not due to a lack of imagination. Or because I haven't heard some cool spy trick you have heard about. It's because I find it (in the absence of other information) the most likely explanation given a lifetime of observations about human nature.