When did people started noticing that people were missing at the Bermuda triangle?

When someone they saw leave, never came back. That happens everywhere, not just at the Triangle. Losses at sea happen everywhere. Where there is a lot of sea traffic, there will be more losses. Berlitz got the idea that one particular patch of ocean was having more than its share of losses (apparently not true). A lot of the losses in the BT books either never happened, are from known causes, or are not known to have occurred in the Triangle at all! Some are known *not* to have happened in the Triangle, but wound up in the books anyway. See Kusche's book "The Bermuda Triangle Mystery-Solved" to see what is really going on there. You can probably get it for little or nothing at bookfinder.com which is where I have the best luck finding stuff.