Is this an add-on system that was installed after you bought the car? Most of those monitor battery voltage and will start the engine automatically if it gets low, to charge it back up. There was at least one incident where a mechanic had the fuel rail apart and left the hood open, then went home for the night. The under-hood lamp drained the battery and the remote tried to start the engine. Fuel sprayed all over, got ignited, and burned the entire dealership to the ground. That is not something that will happen in the short time it takes to change oil, however, you have to understand that hybrids are some of the most expensive cars to own. There are very strict requirements related to which engine oils are acceptable to use, and changing it takes longer than normal. There can also be a cooling fan for the drive battery and that can start up unexpectedly. Your mechanic is worried about the engine starting while he has the oil drained out of it. On some hybrids that will not occur if the key is left in the ignition, but now we have a new gimmick with push button start and no keys.

There is a smaller 12 volt battery near the main battery pack that can be disconnected to prevent engine start-up. Most mechanics don't like to do that because besides the obvious loss of radio stations and clock programmed by you, fuel management data stored in the Engine Computer gets lost. You'll never notice that with most cars, but there's a whole new set of variables to be concerned with on hybrids. If the remote start came on the car from the manufacturer, the people at the dealership will be able to tell you the best way to prevent the engine from starting. There's too many liability issues to just guess which one is right for your car.