Previously the chairman of the committee, Rob Sewell, had said Mr Warren was on leave on full pay for "personal reasons" but couldn't say when he would return to the job.
Numerous attempts to contact Mr Sewell by phone and text message this week regarding the private investigator have not been returned.
One volunteer, who wished to remain anonymous, said they had been told not to comment on anything to do with the centre but said the public who supported the centre had a right to know if things weren't right.
"When it's donations and public money that keep us going then those people who give that money need to know where it is going and what we are doing with it," she said.
She wasn't sure why Mr Warren was not at the centre and had concerns about why an investigation was being undertaken.
"Who is going to pay his bill? That won't be cheap. More money taken away from caring for the animals. It's not right. We give our time because we believe in the ethos of this place but when things aren't going well, we all want honesty."
Another volunteer said she had major concerns as well.
"It needs to be about the animals not personalities ... why [Mr Warren] is on full pay is beyond me. That money was given for the animals. There is no excuse to keep people in the dark. If something is wrong then they need to fess up and tell everyone what is going on. You can't ask people to give money and then have no accountability to them. That's not right."