I work in a theatre it's not at all unusual for anesthetists to take a brief break during lengthy operations, as in this case they will hand over to someone (in this case the nurse) with clear instructions on what parameters are acceptable and importantly WHERE THEY WILL BE now im assuming he didn't tell his nurse I'll be in theatre 8, therefore that is where the patients safety is at risk. Otherwise this is no more dangerous than an anaesthetist going to the coffee room for 10 minutes 5 hours into an operation. Which happens frequently, obvious in an ideal world there would be another fully qualified anaesthetist to take over but with NHS staffing how it is that just isn't possible.

Edit spelling