Maybe it was the weekend and they had early hours on Saturday/Sunday? Your point is valid, especially with modern gyms. However, keep in mind, this was in the 1970's, gym hours may have been different back then.
re: I'm not sure about Gym hours, but Gretchen Corbett can work overtime anytime she feels like it in my book
So great to find another Gretchen Corbett fan! Columbo certainly isn't the kind of show that I watch to enjoy beautiful women regularly, but Gretchen sure does it for me in the bikini in this episode! I've seen every minute of Columbo, and her few minutes in the bikini are my favorite!