I tend to treat pens/pencils like money. I'll pick a penny up off the ground, or even a 20 dollar bill if I happen to find one... if it's on the ground and I didn't see someone drop it, it's fair game... same with pens/pencils. I won't pickpocket them from people, or take them off their desk when they aren't looking, but if someone leaves one at their desk (by which I mean school desk, between classes... your desk at work actually belongs to you when you aren't there and is different) I'll pick it up.
Then again, when I lend out pens/pencils I'm not too insistent about getting them back. They are communal property, and as long as everyone who needs to write something has at least one it doesn't seem to be worth stressing over either way. Then again, I feel similarly about most anything I can't find the owner of... I'll check a wallet, a phone, or a flash drive to see if I can find who it belongs to, and I'll use a set of car keys with the beeper on it to find the car and then return them, but if I see a marble, a die, or anything else that interests me lying about I'll definitely just take and keep it... not really stealing.