Doorbell question

OK so I live in an apartment, today for the second time my doorbell rang I went right away to open it and there was nobody there ? . Like usually when people are there they stay there for a little to see if someone opens or not but there wasn't anybody the thing is this is the second time , the first time was probably a month ago and it was at 7 in the morning and again no one was there this time was at like 1 pm but Idk I feel scared cause I'm always alone with my kids since my husband works . Should I be concerned about this I mean my husband just tries to keep me calm saying maybe it rang on its own but Idk I've never heard of that happening or he said maybe a prank but why would they target our apartment again Idk I just feel very nervous