Okay, provided the requirements of knock and notice are met, police and probation can force entry to a probationer's residence and any room that he has access to. They cannot force entry to a roommate's room where the probationer ostensibly has NO access to.
CA has among the strictest probation requirements in the country. One USSC case stated that we had THE strictest. And, no, the probationer need not be present before we can make entry. We also do not need reasonable suspicion, probable cause, or even a hint of the possibility of wrongdoing to make entry. If you choose to rent to or share a residence with someone on probation with search stipulations (which are almost all of them), you choose to agree to this intrusion. Don't like it, evict him.
In the OP's case, chances are he was told to stay where he was and defiantly ran/walked/fled upstairs and locked his doors ... I doubt it was a casual saunter as the officers hung out downstairs for a few minutes. But, he can argue that one in court. The officers did have the right to force entry to the residence, though they would have to articulate more to make entry to a room that the probationer does not have access to (something they may not have known, or may have had other reasons to enter ... as I suspect the police report will be substantially different from the OP's rendition).