What happened in the past was the 2017 Las Vegas mass shooting [1] - the deadliest mass shooting by a lone gunman in American history. It was conducted by shooting out of a hotel room window. The shooter had 24 guns and over a thousand rounds of ammunition in the hotel room - which went unnoticed partly because the guest put up the 'do not disturb' sign.
The hotel owners have presumably decided that the high-probability-minor-damage risk to their business from a few paranoid types avoiding their hotel is not as great as the very-low-probability-enormous-damage risk from a copycat mass shooting.
And so, ignoring the do-not-disturb sign and snooping on guests' rooms is the norm in Vegas these days.
And if they happen to have a large block booking of particularly privacy-conscious people and so a noticeable fraction of the rooms are declining maid service, I can understand why they'd want to be ready to carry out some supplemental checks.