So when I make the Matrix Perception Action to find any mark on my files/devices what or who will protect the mark from being found?
Marks are typically invisible. For example you can't just glance over all your devices, take a matrix perception test and then notice all unwanted (or new) marks.
But if you decide to spend the time and effort to analyze one specific file icon or one specific device icon (the matrix-equivalent of the Observe in Detail Action) then you are allowed to take a Simple (unopposed) matrix perception test.
Hits here are then used in a game of 20-questions. Each net hit let you ask one specific question.
One specific question you can ask is "please show me all marks that are on the this icon".
Spotting an unwanted mark does
not automatically mean that you spot the owner or the mark. It also does
not mean that you are now 'aware' of the owner and are by that eligible to take a matrix perception test to spot him or her.
...but it
does make you eligible for taking the Trackback Matrix Action on the mark.
Calibrating your filters to see the barely visible wisps of information in the datastreams that connect marks to their owners and not the billions of overlapping datastreams is a painstakingly laborious task and is resolved as a Extended Computer + Intuition [Data Processing] (10+Sleaze Rating of the owner of the mark, 30 minutes) Test.
...and if you have an Attack rating (which typically mean that you are connecting to the matrix via a cyberdeck or a living persona) it also make you eligible to take the illegal Erase Mark Matrix Action which is resolved as an opposed Computer + Logic [Attack] v. Willpower + Firewall Test.
When you matrix perception your own icons, it's unopposed. "Are there new marks since I last looked?" is a valid question, assuming you were able to get at least 1 hit in an unopposed test.
Yes.
SR5 p. 236 Recognition Keys
...security-minded Matrix operators will often have agents or even spiders constantly using Matrix Perception to look for unauthorized marks on sensitive icons...SR5 p. 235 Matrix Perception
When you take a Matrix Perception action, each hit can
reveal one piece of information you ask of your gamemaster.
Here’s a list of some of the things Matrix Perception can tell
you.- The marks on an icon, but not their owners.
...but only as long as you spend one action per icon you wish to analyze.
SR5 p. 219 Matrix Authentication Recognition Keys
Normally, marks are invisible to anyone except the person who placed them. To see other marks on an icon (or your own icon), you have to analyze it....have an agent running in their commlink...
KC p. 27 Agents
Agent programs also cannot be hosted on program carrier modules or the virtual machine program.