Wife has dementia and a person comes twice a week to help her shower, but she is hiding her soiled underwear and we can't find it. Advice?

Look under the matress. My GFs father was hiding things there.

Not to be rude--but don't you have a sense of smell? My mom's depends are so rank they can be smelled from the front door of the house she lives in. She has an 'adult' diaper genie, which is barely better than having a week's worth of soggy depends in the trash, fermenting away. She REFUSES to let anyone take out her bathrom trash, so after a couple days--it's bad. After a week it's almost palpable.

And she tried flushing the depends which causes a major plumbing backup. Even those 'flushable wipes' should NOT be flushed.

Mom will sometimes wear the disposable briefs with a pad in there too. This is for days she doesn't want to use the bathroom at all--she also has a permanent catheter, so I do not know where all this liquid comes from!

And a person who will flush briefs will also flush a disosable adult diaper.

I had this problem with my moms underwear. My mom was flushing it down the toilet! Fortunately, we noticed the toilet was not flushing . The plunger was not working either! We called a plumber they had to run a line with a camera from the inside of the toilet down the pipes which ran under the house , front yard, and under the street in front of house. The plumbers could see blockage in the elbow of the pipe which was under the street. In the end, the plumbers pulled out 3 pairs of underwear and 2 wash cloths. It was a complete mess not to mention expensive! I started locking the bathroom doors. I put a portal toilet in her bedroom to use in the evenings. During the day, I would supervise all activity in the bathroom when she asked to use it. If I left for a few minutes to do something outside during the day the bathroom door was locked. Eventually, we got another portal toilet for my mom downstairs to use as the dementia got worse.
Look behind the head board in the bedroom, the bedroom drawers, closets, etc. My mom would throw toilet paper behind the bedroom head board.

I could get my mom to shower if I told her we were going out--even if it was just for her to sit in the car (covid) while I ran into the store, so that ended up being my M.O.