Some people leave the rooms in a nasty condition and housekeeping has to take care of it, no matter what is in front of them.
People throw up in their suites all the time and leave everything up to the housekeeping.
The staff has to keep an eye out for sex traffickers.
People carry illegal substances a lot of times and the security team has to make a decision about it, depending on the stature of their guest.
You may find bed bugs in your hotel bed so it is always best to check.
Pillowcases are changed only when they start to look dirty.