Block heater fires are not that uncommon, we have had probably half a dozen threads on them over at the Powerstroke forum in the past year. The PSD heater is 1,000 watts, which makes for more current than on the Benz heaters.
Also 7.3 Superduty trucks (99-03) are in such demand that resale value is higher than Blue Book value most eveywhere, and they are easy to sell on top of it, no reason to torch one.

Add to that, if someone wants to get rid of one just park it on the street for a few nights, it'll disappear.

I'd say it's a pretty big stretch to say it was arson.

__________________
Brian
87 300Dturbo 180K #14 head still running R-12 SOLD 12/2017
02 F350 Powerstroke 180K
05 Chevy Express 1 ton w/Royal Utility box 120K
08 Infiniti FX-35 40K
15 Golf Sportwagen TDI 35K
10 Sprinter 3500 chassis with a Class A Winnebago on it. 56K