> BBC incorrectly attributed this to "dark sense of humour of Bhutan"

The headline author did. But the article mentions both the fact BRO is Indian and the fact these signs exist in other Indian border areas.

>> Over the years, the Border Roads Organisation has also built and maintained roads in other countries neighbouring India, including Afghanistan, Burma and Sri Lanka. Today, they largely employ local workers, but historically, many Indians took on the difficult and dangerous work. Their ability to have a sense of humour in challenging circumstances is clear both in their brilliant road signs and in their mission.