You may not know who these co-workers are until they "borrow" something from you. Maybe your co-worker "borrows" something work-related, such as your favorite (expensive) pen, and then walks off with it, forever. Or maybe you loaned your co-worker one of your other personal possessions, such as a new gaming DVD or a kitchen item you brought to the company potluck (my condolences) that your co-worker offered to take home to "wash" for you. Whatever it is, the item is now missing from your collection and increasingly you fear you'll never, ever see it again. How do you raise the issue with this co-worker without sending your professional friendship to the point of no return?
First, let me say that you should ask for the item back because IT BELONGS TO YOU. Second, leaving the issue unresolved could damage permanently your relationship with this co-worker. You'll be sitting in a meeting months later listening to this co-worker offer a brilliant summation of the problem at hand, and all you'll be able to think is she still has my favorite (expensive) serving spoon that she borrowed at the office potluck six months ago and I. WANT. IT. BACK!
Why oh why did your co-worker put you in the uncomfortable position of asking to borrow your personal possessions and then forcing you to hunt them down at a later date? Some borrowers are, quite simply, forgetful. Maybe the borrowed item has been sitting in a semi-visible area of his or her home waiting to be returned, but its presence doesn't register high enough on this distracted person's scale to make it back to your desk in a timely manner. Then there are the people who...well, we don't want to let our minds go there, but we're starting to suspect they intend on keeping our stuff forever and ever. But you still want your money and your little black t-shirt back, stat. And don't forget!
I've been scouring the web for a dollar figure related to workplace "borrowing" and haven't found one yet. Perhaps it doesn't exist. But I'm willing to bet it adds up to a big chunk of change after all of the "borrowed" pens, umbrellas, clipboards, books, DVDs, money and winter scarves have been tallied. Borrowing and lending is going on, all the time, right in front of you at work, and it's quite possibly leading to some very hard feelings and awkwardness all around (to the former co-worker years and years ago who "borrowed" my best pen, you know who you are).
Asking (or asking again...) effectively and diplomatically is the tricky part, though, because having to ask FOR YOUR OWN STUFF BACK can feel oddly uncomfortable. In fact, asking can feel like a confusing, huge hurdle to jump for many nice people, who will search for the right moment to bring it up and then when the moment presents itself, they might shy away from asking. Oh, it's only a DVD, I can buy another copy sometime, the "borrowee" thinks to herself. The problem is, you can't buy some self-respect in the $9.99 bargain bin at Best Buy. This whole thing isn't about the DVD, it's about the principle of the thing. Take it away, Latoya! (Warning: Language NSFW.)
Wikihow offers good advice for dealing with the delinquent borrowers in your life.
Above all, don't be afraid to ask your co-worker for your things back -- gently and repeatedly, if necessary -- and know that the average lawyer would be on your side. Yes, borrowing-but-not-returning something is generally considered a form of theft under the law. Suddenly, your "borrowing" co-worker could be living on borrowed time.
And you're going to be much more careful about letting your co-workers borrow from you in the future, right? Fool me once, shame on me. Fool me twice, can't get fooled again. I just borrowed this line from someone very famous, and I don't have to remember to return it. So, there.