Is it possible to install an app and make it completely undetectable?

[Android Central Question]

If so how?

Also, if someone else had secretly installed an app to run in the background on your phone, how would you go about finding that super hidden app or program?

 

[Rukbat]

Not if you want it to be able to run. Whatever is in RAM is detectable. The average user might not be able to find a hidden app, but anyone who can look through RAM (there are apps to do that) and understand what s/he's seeing will find them.