I believe even if all of the signs "have fallen off" on the entirety of the block in question the tow is still legit.

Was there a period with an unusual lacking of parked cars on your street today? If so it's possible there was street cleaning. There's also a resource somewhere on the city website with a schedule or at least there was last year.

If your car is newer and has a spoofable key fob that you keep near your front door—my neighbor had that experience a few nights ago. How bizarre.