Does anyone know a door alarm system for an elderly person with Senile Dementia?

Hi, I was wondering if there was such a thing? My grandmother has senile dementia and is very forgetful and sometimes gets very confused. We are beginning to worry incase she decides to go walkabouts in the middle of the night. So what Im looking for is an alarm system that can be fitted easily to her front door that can inform us if her front door is opened but without sounding a LOUD audible alarm. I understand a lot of alarms send text messages now, so something like that would be ideal. We live only a couple hundred meters from her and so would be able to get to her in a few seconds. Thanks for any suggestions.