Maybe a water monitor (Varanus salvator). Most monitors are difficult to tell appart for me, but given the apparent size, frequency in captivity and enclosure, I would vote for a water monitor
This appears to be the rare stone monitor (Varanus itsjustastatueensis). They known for staying completely motionless for days, months, or even years. As of the writing of this comment, no one has documented seeing a moving individual.
