First things that come to my mind:
1. Faulty hydrometer - solution: go get some distilled water and check it. If it's bad go get a new one.
2. Faulty thermometer - same solution: check it in some boiling water and some ice water. If it's bad go get a new one.
3. un-heated tun - solution: heat your strike water over your dough-in temp. Let it sit in the tun until it cools down to strike temp.
4. Incomplete dough-in - solution: stir thoroughly as you ad the grain to the strike water. Add the grain slowly enough that you get a good mix.
5. strike water too cool - solution: pre-heat mash tun, and account for the grain temp
I really think it's 1 or 2 and a combination of the rest. I can't see that much/rapid temp loss happening with the thermal mass of the grain unless you leave the lid off and you're in Nome. But each variable brings a new set of problems, and they will compound each other.
But even with all of those things happening, 1.012 is just ridiculously low. That's why I'm leaning toward a bad thermometer
anda bad hydrometer. Thermometers are relatively cheap, so I'd just go ahead and spend a few bucks more and get a good one. Also, look closely at your hydrometer...is there
any liquid at all inside of it? I actually had a hydrometer that wasn't correctly sealed, and it developed some condensation on a cold day going from storage to wort. Moisture (even a thin condensation haze) in the hydrometer = added weight = low grav readings. The usual suspect is a slipped paper insert, so check for that as well.
But if we had vBookie on this site, that's where I would place my money.