It could have been any number of things that indicated murder had been committed, but not necessarily in the room. It could have happened in the car for example. They may have found CCTV footage of her leaving the hotel with him and him returning, and found a weapon with her blood on it - there are any of number of scenarios which I'm sure are less likely than her being killed in the room, but until we know for sure we shouldn't assume. There is still a big question of how he sauntered out of a hotel with a dead body.
Could have been a cadaver dog
Looks like he left the body in place for some time.