I have gotten this twice. Apparently it shouldn't be sent to your phone as it potentially could mean nothing and it disappears. One article said when I googled it as I was concerned and it basically is just to say you have been within 4m of a covid positive. If you have been briefly around someone e.g. walked past them four metres away, you won't be told to isolate as it is little to no exposure. If you have been between 2-4m, it is little to medium amount of risk and they.will assess how long for and the distance. If you have been 2m for longer than 15min, you will be told to isolate in a second notification that will not disappear when you click on it. Of you have been exposed to covid, this notification will stay on the app and you will be told to isolate. So I wouldn't worry as worrying as it may be.