Tip the staff at the location
for looking after the pinball

Tip them coins !!! It will be free

Scorbit looks like it is a promising solution for this kind of problem. Check it out here: https://scorbit.io/scorbitron

Old-school operators know this is a hard problem to solve. The location needs to feel that it is worth their while to have the game running. Then again, old school operators visit every location every week, because one week is the traditional time period for an operator's cycle, as in the typical expression, "That game makes [number] bucks a week!"

Are you open to the idea of putting a business card or "For service, call nnn-nnn-nnnn" sign on the lower apron, under the glass? That way, you could get reports from the players or any interested party.
.................David Marston

What is your maintenance schedule for this location? Maybe you should be out there every week, especially if the game has a history of being down on occasion. If I had a game that was going down on several occasions I would consider rotating in a different game with a better uptime history, or I would be out there every Friday to make sure it was up. Also, there is so much employee turnover with most bars that relying on the staff to help can be hit or miss depending on who is working.

Unless you cover the coin slots, you can get coin jams on any game whether or not it has a DBA installed. I have fixed plenty of coin jams in a pin with a DBA on it. People use whatever they have in their pocket. As far as your maintenance schedule, going more often will catch gameplay issues that some customers won't notice or even think to call about. It helps keep your games clean, too.

Sorry, you really can't expect anyone else to do your job for you. If you want it done right you have to do it yourself. Go in every week.

This person used a Photon board and the IFTTT service to make a power loss detector to send emails when power is lost. You can probably set it up to let you know when it’s restored too.

https://www.instructables.com/id/Build-a-Simple-Power-Outage-Monitor/

I believe the service outlet is always HOT as long as the machine is plugged in - so this wouldn’t catch the scenario of the location switching off the machine but leaving it plugged in, unless the outlet is rewired somehow.

This would also be an issue with your WiFi camera idea if I understood that correctly.

The right thing to do is go in every Friday to make sure machines are error free for the weekend and then go again on Monday to check for problems. You could put nest cameras on all your machines but then you would waste as much time monitoring them as actually going there. Your post comes across as you simply don't want to put in the time. If you want your location to be 100% you have to maintain it.

The arcades at Disney World had these QR codes on them where people could use their phones to report an issue on a website. Seemed like a good idea, just have to wait for a pin player to help out and report the issue.

I know operators in my area visit their games on location weekly.

a simple card or note on the apron saying "to report any issues, or if this game isn't on, please contact me at _____". create an email or facebook page just for your operating biz. any players who care about the upkeep will take the time to contact you. it won't solve all your problems, but it will certainly help. and even better, if you know a problem will require ordering parts you don't have on hand, you can get a head start on that.

That customer is dumb. It's not making them money when it's off. I'd have one simple conversation with the customer/owner stating the obvious and express how you want this handled going forward.
1.) Machine is to always remain powered on unless smoke is seen.
2.) You are to immediately be notified by phone if someone reports a problem. Leaving a business card taped to the back glass. They are to put an out of order sign on the machine (leave on top of back box).
3.) Let them know any further incidents will leave you no choice but to pull your services.

While this is nothing short of the "normal" expectation by an OP, some people are just plain dumb and need things laid out in black and white. I'd also recommend a service log is kept in the machine. If your service is ever questioned you have a log file to prove your innocence.

I have a great player base and staff at locations that let me know about 70% of the time when there is a problem. They usually tell me the symptom and ask if I want it turned off. Even still some players don't bother or the staff gets busy and it gets turned off and not reported. I installed a $30 webcam at each location pointing at the games, it's not super great quality but I can tell if it's turned off or stuck in a tilt or something. I also have wifi surge protection switches/timers for them all so they can be power cycled from my phone if necessary. Best money you'll spend as an operator after DBVs.