macroeye
builders generally store their signs for the next use when a project is finished unless the signs are worn out or damaged. if you're interested, you may approach the site security to seek the project manager to try if they'll give you a sign, since these companies usually buy them at discounted prices...

DA
If you "steal" them, then yeah, it's illegal. The signs are reused. When the builders are done in one area, they pick up, go to another area, and need to use the signs again. I'm a little touchy on that subject. As a former sign stealer, I can see the attraction, even if the signs are useless. As someone that just paid for signage, I can see this being a major pain if someone kept pinching my signs and I had to buy new ones all of the time. As a suggestion, contact the builder and ask them if you can have one of their signs when they no longer need them. I worked in a record store at one point and that's what we did with all of our promotional materials. When corporate said to take it down and discard it, if someone told me that they wanted it before it got to the trash, it was theirs.