Unexpected mildew smell in my rarely used guest bedroom closet—any quick fixes?

We just finished painting and redecorating our guest bedroom, which we only use a few times a year. The closet inside that room hasn’t been opened for months, but when I finally checked it yesterday, I noticed a strong mildew smell. There’s no visible mold or dampness, and I keep the house pretty well ventilated overall. I wiped down the shelves with vinegar solution and left the door open for a day, but the smell is still lingering. I’m worried it might affect clothes stored there during visits. Has anyone dealt with a stubborn closet odor like this without ripping out walls or buying expensive dehumidifiers? Would baking soda or activated charcoal really help, or is there a better quick fix to freshen it up before our next guest arrives?