Feeler gauge

the expression feeler gauge designates:

• a person or a software, who procures in secret information, see to agent
• a wide angle objective built for safety purposes into doors, see to spyhole
• in the broadest sense also different optical devices (z. B. Mirrors) one unnoticed observing make
• colloquially a feeler gage possible

this sidea term clarifying is for the distinction of several terms named the same word.