"It was a strange sight, the guests standing around nervously outside and the completely chilled-out seal snoozing inside," said the association on its Facebook page, complete with pictures.
The seal had become "quite grumpy due to its disturbed beauty sleep" and the animal rescue team placed it in a special basket to move it to a more usual habitat by the beach, the group added.
Despite their relatively placid appearance, seals can become aggressive and bite if provoked, the association warned.
Hotel employee Valentijn Damen told local media Omroep Zeeland: "Just when you think you've seen everything, something like this happens."
Damen said the woman had no hard feelings after reclaiming her room.
"She found it all a fascinating experience. It's also her birthday today, so she thought it was a nice present."