I think that scenario would be rare for sure. But, also, to be fair, I've never seen a Websleuths case where forum users didn't think the fiance/spouse/partner seemed suspicious no matter what they do. Sometimes people think they are over involved and talk too much, but the opposite is true too...sometimes people think they don't show the appropriate amount of emotion or involvement. In both extremes, I've seen the guy turn up guilty...as well as the guy turn up innocent.
People don't all react to tragic news the same way, so it's hard to tell.
I find it suspicious he didn't contact the police, for example, but I can think of rational circumstances that might explain that. Some hypothetical possibilities...he knew she took a trip, they were broken up, she told him to leave her alone, she had a history of extreme behavior recently...again, hypothetical...for purposes of discussion.