Thanks to you I'm not going to have to scan/fire a few rounds at every asteroid I encounter.
Although I'd assume the asteroid ship would give off heat signatures unless actively stealthing, which it couldn't do for too long. Even with passive sensors by the time it saw my whatever signature on radar I would see it's heat signature, so it couldn't mask itself before I knew SOMETHING was out there. However, I wouldn't necessarily know WHAT was out there and would probably be looking for a ship and not a asteroid, so it could/probably would still catch me off guard.
But, assuming blockade runner uses realistic scanning, it couldn't catch people COMPLETELY off guard unless it knew to mask it's signature before they got into sensor range.
/my ramblings
Well, of course they're going to mask their heat signatures! 
And they are most certainly designed to catch you off guard. 
Neice!

Well, yes. But you can't mask it constantly, unless you want to have your crew and equipment fried. Assuming sensor range is constant, you'd have to know I was coming from external sources in order to mask, because if you could see me, I could see you. You could place it somewhere I'd likely go, like an asteroid field I'd like to mine. That would be great for pirating! But once again, by the time you knew to mask your signature I would know something was there.
If it used realistic sensors (it won't lol) then the one moving towards you would see you far before you saw them, so they would have plenty of time to get your location and all before you even knew they were coming. But that can all be rectified by having scouts around.
You have to be really careful tho, if someone knows you are there, they could always just sit outside your weapon range and wait for to overheat and either be forced to vent, showing your location, or fry your systems and then probably scavenged for parts. You have surprise, but it's a double edged sword: the enemy has the control.
I'd recommend moving them around after every successful operation, and not to over-use them.
Edit : totally awesome tactical option now that I think about it. Adds a lot of depth to game-play without being overpowered (obviously). And you could use other ships to pressure your enemy into moving about and not finding said asteroid, or perhaps a far more useful use would be to rather than wait for enemies, lure them to you! That would work spectacularly!