I think it's fine to leave a note, and to ask building security about what happens to it when it's eventually cut off, but not to cut it off yourself. I've had situations where I've to leave a bike parked somewhere for a long time. Especially when I lived in the dorms, and the only options were to keep it in my tiny dorm room or lock it up nearby and hope for the best. One time someone stole my rear tire, and I didn't have the money or time to deal with right then, so it sat there even longer, completely unrideable. If the wheel is tacoed, it's entirely possible that someone had an accident with it, couldn't ride it home, and locked it up to come back for it later, when they can fix it. At some points it becomes officially abandoned, but it's best to let someone with authority (building management or law enforcement) to make that call. Otherwise it's not only morally dicey, but legally as well. Regardless of your intentions and good faith efforts, you get caught riding a stolen bike and it could go badly for you.