Is there somebody in my house or what could be an explanation to this?

I have heard a phone notification noise this morning which woke me up and I believed it to be my partners work phone yet they had it on them at work and not in the house. She thought that I was dreaming yet later in the day we both heard it again. There are no old phones in the house and this is not an iPhone noise which we both have. The house alarm went off yesterday which is also worrying. I have looked in the attic and found nobody / no signs of life. No food etc has gone missing and I don't hear anybody sneaking around. No signs of life other than this phone noise. Any help is appreciated as it is driving me crazy. Especially as my parents don't believe me about the noise.