Clandestine

"Clandestine" means:

1. done, made, or acquired by stealth.
2. acting or doing something clandestinely.
3. marked by, held in, or conducted with secrecy : surreptitious a clandestine love affair

• Sources: https://www.merriam-webster.com/dictionary/surreptitious and https://www.merriam-webster.com/dictionary/clandestine