1. The Sensor Grenades are only activate once they hit the ground and if the enemies are in direct range of it's scanning laser. It doesn't go through walls or anything like.
2. Your teammates cannot see any of the enemies tagged by the sensor grenade. Only you can see them them on your mini map.
3. Most likely, your teammates saw the enemies red dot on their mini map when they fired their gun, heard them fire their gun in the area, and/or saw your skull when you died and knew someone was in the area, but it definitely was not because of the sensor grenades.
Sometimes I run a fun class. One where I primarily use the Combat Ax and just run around trying to throw them at people. I have an RPG only class. So I do run classes that are not designed to keep me alive. However, it really isn't my intention. I just enjoy using some of the weaker set ups. For instance, I really like to use the Executioner. So that is a gun that is really built around getting a kill and dying after 1 or 2 kills (if you run it like a primary without playing conservatively). I also like to spam motion sensor grenades. It is more for my own amusement though. As mentioned earlier, the motion sensor grenade is a personal radar enhancer. Your team isn't aware of who is tagged. So purposely dying to throw more of them out is not practical. Unless you are like me, and you just want to see your "use" number go up.