In a side quest I couldn't find  two hidden items, even though I was in scouting mode,  which according to walkthroughs are there. So my question is what is more important for finding hidden items in scouting mode, attribute like perception or a skill like mechanics.

I'm not positive Ambrusynev but I thought I read somewhere that a character's mechanics level determines whether you can find hidden items, but you must also be in stealth/scouting mode to locate them.  Maybe someone with more knowledge than I can confirm this.

Yes, it's the Mechanics skill and you need to be in Scouting mode.