I’ve been in similar situations but had a lock with me. I moved the free bike a couple of blocks away, locked it to a pole, headed home, and retuned for the bike with my car. And a couple of weekends ago, about a mile into my ride, I spotted a frame at the curb with a “free” sign. I figured I’d check on it at the end of my ride. If it was still there, I’d grab it. It was; I did.