Why werent they detected?

[AYDENSMUM]

Hi all.
Im fairly new to Avast!, so please bare with me.
I scanned my computer yesterday and it revealed 2 trojans. Can someone please explain to me why these werent picked up without me having to scan...surely Avast doesnt let everything through and scanning is the only option for detection??

[Lisandro]

When did you install avast? Aren't these files there before you install avast?
Maybe the detection of them was added just recently and not when you installed avast.
Maybe your Standard Shield sensibility is not set to High...

[DavidR]

avast is continually adding new signatures so as Tech said they may have just been added.

However, without information it is impossible to say.

What were the trojan names ?
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ? 
Check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections.