Somebody must of lost them during, one of those, tragic boating accidents
http://www.9news.com/story/news/2015/08/05/grenade-detonated-boulder-county-lake/31149091/Water level had dropped and a couple walking on the shore found a grenade, then a news reporter covering the "story" found a second grenade.
There is a youth fishing event this weekend, at this lake. I'm thinking they may have a lower turn out than they expected.
Here is the Boulder County Sheriff contact info in case they find anything else that may belong to you

Sheriff's Office
Main: 303-441-3600