If they are close enough to take it and it's unlocked... yes. If your vehicle has been started by Remote Start, from the fob, and idling for about 10 minutes, and you are out of range with your fob, then the vehicle should shut off automatically. I tested that theory to be sure it was around 10 minutes... it was. Remote Start your Blazer in your garage, then walk away at least 50 feet from it with the fob, and let time expire.
If you start your vehicle by pushing the button on the dash, then the vehicle should not shut off after ten minutes and stays on indefinitely. If you Remote Start, and get in without the fob, then you cannot even shift it into gear and move the car. You have to have the fob within range to shift it into gear. Actually, once you get into the vehicle, you have to depress the brake pedal and then push the button on the dash to enable the normal functions of the car.
If you push the button on the dash to start, get out, and leave the fob in the house away from the vehicle, you can still drive it normally without the fob. However, the second you put into reverse, or in gear to move it, you will hear the chime, and a warning comes up on the cluster notifying you the key fob is not in range. So, there is no way to drive the vehicle and not know your fob is away from the car unless you just aren't paying attention... that happens. Check in your vehicle settings menu to be sure your chime volume is set to a reasonable level to hear it.
Yes, someone could jump in your vehicle and take it if it is idling, and not Remote Started while you're away and the car is not locked. You can start your vehicle and lock it and walk away and then no one can jump in and take it.
I tell you what I do that removes the risk of your concern from happening. I set my doors to auto-lock once I leave the vehicle. This forces you to have the fob with you to get back into the car. If you don't have the fob with you, the car stays locked, thus reminding you to go get the fob.