I am 1st owner of brand new, single-story house on a slab. I get 'wet-rusty-pipe' smell in 5x5' utility room, but ONLY when the outside temperature is 80 degrees +. Otherwise, no smell. W/D are hooked up in U. room but only used once about 3 months ago. I've 'rocked' the washer and no sound of sloshing water. No smell in any other part of the house, including 2 bathrms about 8' and 12' away from U. room. No problems with tub drainage or toilets/water levels. This little room is in the crook of the L-shaped house between attached garage and house. No smell in garage or pipe-entry points in U. room, even when the actual room smells. There is an air-conditioning vent and an outside fresh-air vent in the ceiling of U. room, but no smell when either is on or off. I looked at the 'crook' of the L on the outside of the house, which is a washed-rock garden area. There aren't many plants there, so not much watering. I dug back the rock and pulled up the weed-barrier and in the corner was very wet mud. I left this pulled back for about a week now and have been VERY careful with watering in this area and it seems to be drying up at a very slow rate. Looks like drier mud now. Any ideas?

See if the trap on anything in the utility room is drying out. That's the J, P or S shaped thing that should exist on most drains.

Is there a drain in the utility room?

How about a french drain or a large gap between the walls and the floor?

KISS is on to something here.
If you don't use the washer for months be sure to fill the trap. Also, every washer has a sump where water collects and stays. Sniff the washer and if it smells cycle it and then pour a quart of bleach in and let it set over night. Next morning cycle the washer again to flush it out. If you have a laundry sink fill that trap also. Good luck, Tom