Did you by chance have it done at a certain shop? Maybe not finished with payments or labor?
Pissed off anyone recently?
Without a doubt it's someone who knew about the swap and knows where you live directly.
But like the few of us whom still question for what reasons and terms was our cars chosing to be stolen for, still ponders our minds. The only true thing we can do is live and learn. Doing things differently is certainly going into effect.
Mine was stolen for the rims, the cf hood & intake. They left the swap, the exhaust, & angel eye one piece headlights. But then they set fire to the car. The body kit was melted on the car along with the cf mirrors and the nitrous bottle. They even were so kind to leave the plates on the car. Horrible site to see tho. Things we learned... it wasn't fully covered or appraised for a certain amount, no alarm or anti-theft agent, and things were left in the car where they were visible. There's a happy ending to my story tho. They caught the 3 guys that were invovled in stealing it, thanks to blood found on the glass left in its parking spot and finger prints on the motor and fenders. Plus being previous auto theft offenders and drug trafficing offences helped too. We came back into the scene with a vengence with 2 hatchies and used the motor swap in one car and the exhaust on another, to achieve our goals and proceed with caution for the future. Having a savings account does wonders for those rainy days too. 8)
Sorry for your loss and never fear to rebuild.