A skeleton key is a bit key (the type made for antique lever-tumbler locks, which are now considered obsolete and not secure) which has been specially modified for the purpose of attempting to manipulate large groups of locks rather than a single lock. In the early part of the 20th century, skeleton keys were carried by thieves. To this day, it is still a felony to send one through the US Mail. But they have little practical value, except for the occasional bedroom door, roll top desk, or armoire.
Lasciel and GaryM appear to be talking about a mechanical key or an emergency key. That’s not the same thing.
Most automobiles have some method for you to lock your valuables in the trunk and/or glovebox and still allow a valet to drive the car without accessing those areas. Typically, this involves two different keys, one called a master key (which operates all the locks) and one called a valet key (which operates the doors and ignition but not trunk or glovebox). You lock your valuables in the trunk with the master key and give the valet key to the valet. But every car is different. Check your owner’s manual. The catch is that most car owners are completely unaware of which key is which, and often one key or the other gets lost so they only have one but not the other.
A further complication to this is the fact that the vast majority of cars sold in America during this decade use transponder keys (which contain a tiny radio, whose signal is required to start the engine). Remote fob contain mechanical override keys so that you can unlock the door even if the car’s battery is dead. But the mechanical override key by itself will not start the engine because it lacks a transponder. Again, check your owner’s manual. It may say that there’s a way to lock the trunk so that the mechanical override key is needed to unlock it again and the remote alone won’t do it. If that’s the case, then keeping the mechanical override key might keep a valet out of your trunk if you follow the procedure in the owner’s manual.