Let's see, Powell endorsed Obama twice. He came out in favor of the Iran nuclear deal which Clinton was instrumental in crafting. He called out Trump for being anti-immigrant and taking the party much further to the right than the country as a whole. 

Connect the dots. I wouldn't be surprised if he endorsed Clinton before the election.

http://realtimepolitics.com/2016/05/25/republican-colin-powell-burns-donald-trump/