The 75-year-old was last seen at the Pinezanita RV Park and Campground in Julian, California on October 4. Her husband noticed she was gone when he returned from a shower and couldn't find her.

"As you can imagine, they're pretty distraught," said Mustaine about how his family is dealing with the disappearance. "My wife is very, very upset. I think, with [my mother-in-law] having Alzheimer's, it complicates it so much."