Has there been any cases of bitcoin being stolen without getting access to the owners private key?
Yes, for example, a company named Bitcoins Reserve lost 100 BTC when an attacker hijacked the CEO's email address and sent instructions to the CTO requesting a client withdrawal while instead providing their own Bitcoin address.

This and other incidents are described here: https://bitcoinmagazine.com/articles/cautionary-tales-bitcoin-security-1422491521/