I have to undercut your argument here
There is one who works under the cover of darkness and by all accounts is doing right.
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Actually, he's a vigilante who doesn't care about the law. That's why they call him the DARK knight. While you can say there's an overarching "right", Batman is still a criminal based on the laws. Unless you've got the resources of Bruce Wayne, you will get caught and it will be bad for your business.
Aside from comic book characters, there are some practical reasons to repo something at night, assuming it is legal to do so:
1) Avoiding unnecessary interference by others to a legal action - do you really want them to call the cops and spend more of your time explaining, showing documentation, etc.? Or, there's always the case that they DON'T call the cops and decide that they are Batman. Also bad for your business.
2) The shock and awe of seeing your sign disappear overnight can have quite a motivating factor.
3) You get to charge them for a second install to put it back up, which recoups part of your expenses.
On the other hand, I think there's some advantages to repo-ing in full sight/presence of your customer.
1) Your de-installers show up with truck and equipment to take the sign down, and they pay up before you even touch the sign.
2) They cannot accuse you of acting illegally - if the cops come out, you'll have full documentation, and they look like a fool in front of the cops.
3) Taking a sign down is a very visible act, usually. It will get people talking about why a new sign is coming down. And it will all be negative talk. Which might influence them to pay up faster.
Never had personal experience in this area, but that's my thinking through it. The main thing is whatever you do, make sure it's legal first. Don't screw up your business further, and especially don't give them a reason to take you to court for their lack of responsibility.