I would imagine that on most days during the season at a large resort there are quite a few cars in the parking garage?

Remember, the resort did their search of the lodge area's parking on Feb 17 when the SOS was spotted (and only 2 days after they skied off area). However, their family did not file a missing person report until Feb 23. Also, they had not yet checked in to a hotel at the resort. So when the resort did their check on Feb 17 they really had no certain way to tie a car left in the garage to an SOS spotted in the snow.

There are lots of reason people might leave a car overnight in a ski resort parking garage. (Folks who drank too much apres ski and decide to take a cab back to their hotel? People who hooked up with a member of the opposite sex and ride back with the other person?) A car left for a day or too in a parking garage is certainly a clue, but I don't think that was quite as much of a "screaming red flag" as you assume.