If one is in a place on Shabbat where an alarm is activated when the door opens, do they need to prevent other people from opening the door if they are definitely unaware that the alarm is active?
The Gemara in Shabbat 72b says that if one had the intention to pick up something that is not attached to the ground, for instance a knife, but they accidentally picked up something attached to the ground, such as grass, as well as the knife they are exempt because they had no intention of picking up the grass. This is called mit'asek. However, if they had the intention to cut something that wasn't attached to the ground, but they cut something that was attached, Rava says they is exempt, and Abaye says they are obligated. We follow Rava. Based on that, if a person is opening the door and is not aware an alarm will be activated, they are exempt.
The question is whether one needs to prevent others from opening the door. Rabbi Akiva Eger says that despite the fact that one is exempt it is still a sin, and since we must prevent others from sinning, we must prevent people from opening the door.
However the Makor Haim says that one is not committing a sin because one is not aware of it, and so we do not have to warn people.
It could be that Rabbi Akiva Eger would agree in our case that it would not be considered a sin based on the way he learned the Gemara in Bava Kama 26b. There it says that if a person was walking in the public domain and there was a pebble in his armpit that fell that he was unaware of, he has not committed a sin. Rabbi Akiva Eger understands that he is completely exempt; it is not a sin. The reason there is because he didn't know about the pebble, and the result was different from what he intended.
We can also add that here it is called Pesik Resheh because the person is not directly turning on the alarm. He is opening the door, and a side effect is that the alarm goes off. It is also Lo Niha Leh, meaning it is a result that he is unhappy with and didn't want. Therefore, if there is a good reason not to tell them about the alarm we can be lenient and not tell them, especially if there is no other entrance that can be used and if it prevents Oneg Shabbat and Simchat Yom Tov.