It could depend on the hotel, but business casual is what I like to wear.
One thing that I want to do is to blend in and go through the lobby unnoticed if possible.
If you walk into a 5 star hotel wearing raggedy jeans and a tee shirt, you may stick out.

It was unusual if I wear business casual?

Always neat & clean.  Minimum casual sport clothes.   I have worn a polo shirt with clean jeans to a suburban hotel.  In a business environment, a sport coat or suit.  
skb

wear a casual shirt and shorts during most of the year.  In our short winter, jeans, maybe a flannel shirt over a t-shirt, and jacket if needed.

Swim

Sunday's Best.

JUST KIDDING!

I wear what I wear to work usually.  Business casual.  Button down shirt, no tie.
wool slacks and and dress shoes.    When it gets really humid in Boston, I show
a  little bit more chest hair, but I never wear shorts.

Occasionally, I'll bust out the mock turtle necks and Zegna suit.  But only when I'm feeling CEO.

BUT ALWAYS:  the best socks money can buy.