Occasionally you may see some nasty red dots nearby, but mysteriously no enemies. What's going on? Well, unfortunately the tracker seems to take no account of vertical distances. Those alarming 'phantom dots' are likely to be for enemies either way up above you (e.g. on a bridge you crossed earlier in level 5, if you left some of its occupants alive) or way down below you (e.g. indoors in level 4 where enemies lurk on many levels). Phantom dots may also be Flood spores that are hard to see. It would've been better if dots faded with vertical distance or something, but maybe the UNSC boffins will fix that later.
Faster dots for Elites
You can potentially tell Elites apart from other enemies in your tracker, because their dots tends to move faster. This can be a useful warning. Of course, once enemies get sufficiently close, their unique sounds can be used to identify them; but you may not always be close enough. A case in point is in level 3 when you arrive on the 'Truth and Reconciliation' and unseen enemies filter towards closed doors. From the centre of the room you can tell when an uncloaked Elite is among them because there was a fast dot on your tracker as he galloped up to the door. You can prepare accordingly. It's a useful tactic when braving the boarding challenge in that hectic room.
Bigger dots for Hunters
Dots also vary in size. The dot for a Hunter is noticeably bigger than for the other troops, so it's quite easy to tell he's there, even if you can't hear him clanking along yet.