see this also.. Read the indications..
https://en.wikipedia.org/wiki/Trazodone(wikilink is not cos i trust it, but it has stated half life of drug to be 7 hrs. That means that after 7 hours, no trace of this drug will be found in a person's system. Next to be analysed is the breakdown stages of the drug and the rate of synthesis
what time was the drug injested? what else was in the system?
Iam now beginning to wonder whether the beating was used to cover up a different kind of murder?
also it should be remembered that not ALL known drugs are 'tested for' in autopsy. There may have been others which were simply not detected
http://www.webmd.com/drugs/2/drug-11188/trazodone-oral ( better link, as in more reputable for medical stuff)