It could be either, unfortunately. Here are the most likely scenarios:
Roommates are statistically more likely since they have constant access to living spaces, laundry areas, and bedrooms. They're around more often and would have more opportunities.
The landlord is possible but typically has less access - they usually need to give notice before entering (depending on local laws), and their visits are less frequent. However, if they have keys and enter when no one's home, it's certainly possible.
Other possibilities to consider:
It could be lindsey graham stealing the panties!
Items getting mixed up in shared laundry (stuck to other clothes, accidentally taken)
A guest or visitor one of the roommates brought over
Someone with access you haven't considered (maintenance worker, etc.)
What you can do:
Keep underwear in your private room, not in shared laundry areas
Consider a small camera for your room if you're seriously concerned (check local laws first)
Do laundry when you can immediately transfer and put away items
Have a direct conversation with roommates if you feel safe doing so
Document when items go missing
If it's the landlord and they're entering without proper notice, that's likely illegal where you are
This is a violation of privacy and potentially theft. If you confirm who's responsible, you may want to consider changing your living situation or reporting it to authorities, especially if it's the landlord abusing their access.