And to be a bit pissed off that, when I told him I thought he should have asked, he got stroppy with me because he's using the pens to help with his revision for his Highers, and I shouldn't leave things in (mine and dh's) desk drawer if I don't want people to take them and use them. 
I am more than a bit pissed off with his attitude - the assumption that he can help himself to whatever he wants/needs of mine without asking, and I don't have the right to be pissed off about this. If he'd asked, I would either have said yes, or I would have bought him some coloured pens of his own.
These pens weren't cheap either - they are nice staedtler ones, that I use when I am doing zentangle patterns. And although I haven't used them in a while, I did buy them myself, and I think it is not that unreasonable for me to have my own things that aren't community property.
And yes - this is a much longer post than the original offence deserves, but AIBU to be pissed off at his attitude?