I have a chest hidden behind a false mechanical wall and in that chest are magical items, would the player using detect magic be able to notice the items?
It depends on what material the wall is made of.
Detect Magic can penetrate certain materials, but depending on the thickness and type of the material, the obstruction may block its path.
The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.
So if the wall is either:
- Made of metal, and less than 1 inch in thickness, or
- Made of stone, and less than 1 foot in thickness, or
- Made of wood or dirt, and less than 3 feet in thickness, or
- Made of anything else, except for lead,
then the spell can detect what's behind the wall.
Depends how thick the false wall is and what it is made of. From the spell description:
The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.
Also, note from sage advice:
Jeremy Crawford @JeremyECrawford
Sensing a presence means you sense something is present, not its location. "Wait, magic is nearby . . ."