As a general rule never loan knives nor leave them laying around for someone to nick. Given that you look after the tools you own, then accept that no-one will look after your knife better than you do. Loan with reluctance and only to those you can trust or do not loan anything at all! Better still, use your own knife to carry out their job for them. People have come to me in the past to re-sharpen and restore what had once been perfectly good knives, but were in a sad condition due to neglect and misuse. Many admitted 'a bloke borrowed it and brought it back looking like this'.
I recall an American-made Gerber LMF knife being brought to me by the owner to resharpen. The LMF was a heavy-duty survival-style model that was quite popular in the early 90s. I found it hard to believe that a blade could be so harshly treated as this one had been, particularly since LMF blades are hardened and tempered beyond 60 Rockwell Hardness (RC).
Restoring the edge, as far as I was concerned, also meant restoring as far as possible the blade as well. And therein lay a problem: having to restore the facets of a very hard blade without totally dismantling it to do so. Deep abrasions that remained at the end of the clean-up became 'ageing marks', partially masked by a hand-rubbed satin finish using 600-grit wet and dry paper. With care, I managed not to obliterate evidence of the manufacturer's trademark. Then I set about the task of reproducing a shaving-sharp edge on the LMF by re-honing to the correct angle before polishing the edge to razor-sharpness.
True, I was only required by the owner to re-sharpen the LMF; however, once I took on the job there was no way the abused knife was leaving Spencer's workshop in sharp, but shoddy condition. I accepted a modest fee for sharpening the LMPJ the job he asked me to do. The smile on his face when he received the restored knife was payment enough for the rest.
People who forget to return things loaned to them in good faith are annoying because they have to be chased up to get those things back. Anyone who steals your knife deserves to be lashed! Knife theft happens more often than you imagine. In the course of running the a local American Knife Collectors Club (AKC) over the past 14 years an alarming number of knife users, makers and collectors have shared with me stories of their stolen bladeware. Through my own carelessness, it once happened to me too.
In addition to being mindful of where you leave personal bladeware, how else can you protect yourself from the light-fingered lowlifes that lurk around in society looking to purloin someone's prized possessions? Make your working knives as 'unpinchable' as possible. Mark them or customise them in ways that make them easily identified as yours. For example, even a factory-made Buck 110 Folder, of which there are thousands about that look exactly the same, can be distinctly marked in a certain way. The potential thief can't use it around anyone who might identify the mark, nor can it be shown to another collector, who will immediately want to know more about any markings on a knife, a collector's trait. So the 'tea leaf' is stymied.
Be careful, though, not to make the markings on your working knives too fancy (for example, scrimshaw), otherwise it might make the knife more 'stealable'. Discreet but distinguishable filework on the back of the blade and contoured handles are good because they are permanent - they cannot be removed or revamped. A grinder can easily remove blade etchings and bolster engravings. Of course, if the knife has been custom-crafted it should come with discernable differences associated with one-of-a-kind bladeware. Still, even custom-crafted working knives that are more functional than fancy, should be in some way especially marked for individuality. Then they should then be photographed and recorded.