London Zoo says it believes Melati carried the cub outside, but keepers are unclear as to how the cub got into the pool.
Keepers are now reviewing the situation because it was not forseen that the mother would take the cub outside so early.
More top news
A senior employee at a 4-star London hotel has been jailed for six years for raping a man who he lured to a fake job interview.
Children's charity Kids Company is being investigated by the Metropolitan Police over criminal allegations.
The new list is the first to be published since the general election. Some of those identified owed money to a handful of workers.