Vovochan, simple. Common known explosives are detectable by equipment so designed to detect them. This is true of any detection system, they are reactive.
The shoe bomber evaded detection. The liquid limit again came in after first use
Making phones and laptops power up was yet another.
Now I know two common similar objects could be designed to by pass security ONCE. I know another common item that could be redesigned for several purposes but still pass the scanners.
And a friend showed me another common item that 99% carry but is lethal in trained hands.
These devices are so far undetected but are never the less known about.