Question: I left non mevushal wine in an open brown bag on my desk in school overnight. There are non Jewish janitors that clean the room each day after everyone leaves.
Can I use the wine or should I throw it out?
Answer: If the wine is still sealed, or if it certainly wasn’t touched (and seen) by a non-Jew (i.e., it is clear that it remained inside the bag the entire time), it is permissible.
Otherwise, if it’s known that the wine was touched by a non-Jew, it should not be used and it must be discarded.
If it was merely seen by a non-Jew, it is permitted, but some have a stringency not to use it for kiddush or other kos shel bracha. Many chassidim practice this stringency. Others are particular not to use at all.
See also here.