A vehicle or personal tracker has a green symbol when it is online, it becomes red when it hasn't transmitted any data for more than 5 minutes. This is an indication that there may be an issue with the unit. Some devices have a much longer interval (e.g. 1 hour), especially when they are stationary, to save power and/or data. These units appear to be offline, but they're not.
There is already an option to set a "no data received" email notification (in nr. of minutes), which seems to work well. Why not link this timer value to the symbol color time out?