I work in a field where I always work with an apprentice and we frequently work with other crews, and sometimes we work with or near other trades/subcontractors. I mark all my power tools and batteries very conspicuously with yellow paint pen, as it just makes it easier to recognize what’s mine vs not mine when packing everything back up.
Almost everyone I work with carries 1-2 pairs of knipex cobras or alligators, and a wiha 3 - 4 mm screwdriver at all times, and so those get some yellow too.
As far as “hidden” markings; on my personally purchased higher-end hand tools, like my proto Ratchet wrenches, knipex bolt cutters/dykes/snap rings, t&b crimpers, Etc. I do something a bit more permenant and very discrete..
I take a file, and put two very small dashes “=“ through the finish, in the same spot on every tool. It’s invisible if you’re not looking for it, as it would almost look like normal wear. However, if you were to look closer, it is clearly deliberate, and if you looked at it side x side another 5 of my tools, it would make a pretty strong case that the markings were made by the same person (me). I only started doing this after working on a non-union job and had a pair of nice pliers disappear from a cart while I was working just a few feet away out of sight.
Yes, it damages the finish, but I tend to take good care of my tools, and after I make the marking, I’ll generally put some oil or fluid film on the tool.
Tool theft is beyond rampant here in the Bay Area, and my trade is known by tool theives as being “tool rich”, if you will. So having my company name/trade plastered on the side of my truck makes me a target, and the likelyhood of a police recovery is roughly 0%.
I have photos and documentation of all the tools I have in my truck, not for recovery purposes, but for the insurance adjuster should someone steal the truck and empty it out.