I think he may have been.
Was it an accident involving 'black ice'...or a heart attack? I say foul play.
I run the largest and BEST Outlander facebook page (OS) and I posited this theory that I wanted to share.
Being on #TeamFrank is a lonely place...but this may make you inclined to join...or at least give you pause to rethink what you thought you knew.
Here is my post...let me know what YOU think in the comments section below or on OS.
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January 07/ 2017: OS Facebook Post
I think Frank was murdered.
If you have read 'Leaf' and know of his being an MI6 British Intelligence Secret Agent. (the same as 007) and of Roger's father's [Jeremiah] Time Travel experience etc... you will know that Frank was a loving, kind, thoughtful man who felt it all and suffered deeply for his part in trying to save his Country, and the lives of so many others, during the war.
You will know that even after 2 years of not seeing his Claire, he touched his ring lovingly when he thought of her. You will know that he may have had some inkling into the TT and this opens an important door into why Frank may have been protecting Claire and Bree (who are TT and therefore worth kidnapping and examining/using for evil) and why so many red herrings could be employed in the sad second half of their marriage-story together. It would serve Frank well to have Claire believe what she wanted and not get too nosy in his comings-and-goings and late night calls etc....
Any MI6 agent...and especially the top dog like Captain Frank Randall, would be privy to any danger Claire and Bree would be in and perhaps, just perhaps, he felt they were in grave danger when he wanted to take Bree away...KNOWING Claire would follow.
I think Frank was murdered.
If you have read 'Leaf' and know of his being an MI6 British Intelligence Secret Agent. (the same as 007) and of Roger's father's [Jeremiah] Time Travel experience etc... you will know that Frank was a loving, kind, thoughtful man who felt it all and suffered deeply for his part in trying to save his Country, and the lives of so many others, during the war.
You will know that even after 2 years of not seeing his Claire, he touched his ring lovingly when he thought of her. You will know that he may have had some inkling into the TT and this opens an important door into why Frank may have been protecting Claire and Bree (who are TT and therefore worth kidnapping and examining/using for evil) and why so many red herrings could be employed in the sad second half of their marriage-story together. It would serve Frank well to have Claire believe what she wanted and not get too nosy in his comings-and-goings and late night calls etc....
Any MI6 agent...and especially the top dog like Captain Frank Randall, would be privy to any danger Claire and Bree would be in and perhaps, just perhaps, he felt they were in grave danger when he wanted to take Bree away...KNOWING Claire would follow.
And perhaps he was killed trying to protect them. This is obviously just my speculation ... but one theory I am warming too.
He was too important in the war and was the first to reveal proof about the Nazi concentration camps which drove the Allied Forces more passionately than ever...and that made him enemies. And if he knew about Roger's father's TT...which he may have as he was the one who sent him on that mission, it would make him even more enemies and people who would want that info.
I think Frank deserves a noble death...one in which he died for the ones he loved...and I believe (hope!) DG will give him one.
Frank is a man that people think they know, but they don't... not even close. And I LOVE Diane for making this a mystery to us...but one I hope she solves for us! 
ADDED Feb 24 2017: OMG!!! LOOK AT WHAT DIANA HAS WRITTEN ABOUT FRANK'S DEATH!! I HAVE JUST BECOME AWARE OF THIS...POSTED ON COMPU-SERVE MARCH 12 2016:
NOT 100% PROOF...BUT HOLY SHIT!!
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The base for this theory: for further reading on what other 'proof' I have that makes a solid holding for this theory, read here: OUTLANDER: Frank Randall's Lipsticky Collar, Hidden Love Note, Women Callers and Purported Affairs/Indiscretions/Confessions EXPLAINED!
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