Low power circuit that powers an LED but should not shock the human user
At 5V you are already in pretty safe territory as far as human contact is concerned. A representative standard for electrical hazards is ECMA-287:. 
You can use a series resistor of say 10K or more to limit the max current to less than 0.5 mA even in the event of very small skin resistance (which would only happen e.g. with an open wound). The resistor would keep you in the range of ES1 level exposure even in the event of "abnormal" conditions. You could in principle go much larger with the resistor, like 1 Meg or more, depending on other sources of noise. E.g. is there a large motor motor nearby? 

I would recommend using a MOSFET to turn on your LED and/or audible alarm. You should protect the gate of the MOSFET with a Zener diode.  

If we suppose the bottom plate is at ground in your figure and the top needs to be energized to +5 V then a reasonable circuit that would fit in one box could be: