Greetings! While it certainly seems to be an interesting phenomena and may point to the paranormal, are there any other experiences that your mum can relate to other than this smoke detector? As to the history of the house, is it old? Is it on land that was once something else, i.e. Native American sacred space? Is this house built on the foundation of a previous home? Is it on a fault line? All of these are questions that need to be answered prior to looking at further testing to say whether a place is haunted or not.
Certainly, I think I would start with getting a new smoke detector and seeing if the same thing happens. You can then rule out a faulty detector. If it happens, see if whatever is setting off the detector can make it go off on request. This could be indicative of an intelligent type of haunt, i.e the entity wants to communicate with you or, in this case, your mum. Regardless, it doesn't sound like this is trying to hurt anybody. However, a quick solution is that if it goes off in the middle of the night, just tell it to stop it. They generally respond. There could be a reason why this is occurring all of a sudden as well. Are there any issues with the house that need attending to? Many, many reasons.
