Well.... No one but a chopper pilot with a toy like that can tell you how it all works. Just suffice it to say your area has been compromised and you should try and plan for the best methods against such intrusions of privacy. Here's what I believe to be true about the infra red issues:
First of all, we live in a very technological world, but you already know that. This equipment is not cheep, but how many billions go to this BS war on drugs? The best equipment can see right through your attic, second story, probubly into the basement. Most houses are made out of wood, and the floor space is fairly thin, and rarely filled with thick pink insulation (almost never). Believe me when I tell you they could see you passing a joint to your friend sitting in your living room. And yes, by the way you hold and past it a bunch, they would know it to be a joint.
Now, I have seen IR proof items for sale on the hydoponic websites, and you could make your own, if you had access to a very large, thin sheet of led. I have heard of these items being large enough to cover a cealing and an outside wall, or as small as the outside of a small closet grow tent or something. But concider this: A blank spot would certainly show up like a sore thumb just as much as a pot garden.
Now to the question at hand - can they see the weed? YES!
This will probubly be debated for some time, as this sort of subject is very contriversial (spelling sucks I know). Alot of people will say it's a BS rumor, whatever. We live in a high tech world now, right? Well I just think it safer to go ahead an assume the worst, so here it is:
It is widely believed that IR is so good and fine tuned (no longer just a hazy mass of indiscribable pics of heat) that they can see the heat sig a pot plant gives off, not to mention a HID light. OK, like I said, this stuff is expensive, and it's anyones guess if they spent that much - but lets just assume they did. The other thing they are gonna get you on is the arrangement of your grow area. People don't line up houseplants in a neat row under a HID, especialy plants that branch like pot does.
Anyways, I would suggest the basement - with a heat shield on the cealing so they can't see below ground level. It would make more sense then heat shielding a room or closet above basement levels. Be careful - you already suggest they are looking for people in that area - so they must think they know something...