Be careful about HEAT in such storage as well.

I've had to replace the entire keyboard actions on quite a few units for customers because the things were stored in places where ambient heat buildup took its toll on the plastic and weights underneath. Hot cars and vans being numero uno on that list. A closet in a garage might also get hot enough in the right circumstances. Find out what the ambient temp inside that closet might get up to before thinking it is safe to store the keyboard there.

And the closet won't keep out all the salt air, either. It surely would reduce the amount, though.

If going with the plastic bags, I think it would be a very good idea to invest in some of those humidity-absorbing packets to put in there with the gear as well. The kind that you can rejuvenate in the oven periodically.

Of course, the rule of thumb for *any* musical instrument is that it shouldn't be left anywhere you yourself would be uncomfortable or could not survive...
_________________________
"Keep listening. Never become so self-important that you can't listen to other players. Live cleanly....Do right....You can improve as a player by improving as a person. It's a duty we owe to ourselves." --John Coltrane

"You don't know what you like, you like what you know. In order to know what you like, you have to know everything." --Branford Marsalis