For the sake of my sanity in shared shower situations i ve chosen to go with explanation 1.
Yellow streaks on walls in bathroom.
If you can see yellow patches in your home it might be yellow mold.
Visual cues can also be indirect.
Even with well ventilated bathrooms streaks in water based paint is still very common.
The streaks are not all that bad but it is annoying for me who loves keeping my walls fresh and clean.
Those drip mark lines on your bathroom walls do exist.
It makes it even easier if you turn your shower on close the door and let the steam from the hot water build up the humidity in the bathroom with the fan off.
Depending on the species the appearance can vary from bright yellow to muted from powdery to slimy.
Step 1 fill a spray bottle with 3 percent hydrogen peroxide which is found in drugstores or convenience stores for a minimal price.
Typically these streaks have a slight yellowish tint which is released from the paint ingredients.
These lines result after the water evaporates.
This will bring out the spots and streaks even more as well as soften it all up for easier removal.
You re not imagining things.
Wipe your walls down after every shower or bath.
Get rid of nasty yellow stains in your plastic bathtub using hydrogen peroxide.
The second explanation is that the yellow stuff is you eww or more specifically the oils from your skin lifted off by with hot water and redeposited onto the walls after the humidity in the room dies down.