WTGO COIL Leak

someone might have made their own gasket such that it doesn't stick out or used permatex or gasket shellac.

tighten the bolts?

There should be some sort of gasket there, although it might be thin. Sort of off topic, but did they even use the correct bolts? I've always liked to have three threads visible — but that is automotive and aircraft, so maybe it's OK here?

I’m not a fan of that installer. Whoever connected the flexible tubing directly to a 90° adapter, without providing any practical way to service the coil, did a very poor job.

I doubt there is even a mixing valve of any type installed on that tankless coil. If you want to do right by the customer, you may want to show them the manufacturer’s instructions for connecting the domestic hot-water piping. You could then quote a price for replacing the gasket and reconnecting the piping properly, including the installation of a mixing valve.

EDIT:

This is from the instruction manual regarding how to connect the potable water piping. I have added some valves in color and also painted the mixing valve green in order to understand and see it easier. That part is not included with the boiler and must be purchased separately, and installed as indicated in the instruction manual.

This information is on page 23 of this manual: Weil McLain WTGO boiler.

Here are the photos of 2 boilers with a visible red rubber gasket. the factory often uses a black rubber gasket that is a little thinner.

I like it when the copper piping is connected directly to the tankless coil, with enough support for the mixing valve.

The problem I have with using a 90° male-to-copper adapter is that you may not be able to tighten the threads sufficiently while still having the copper leave the coil in the desired direction. That often leads to leakage at the threaded joint. It may not produce visible water droplets because the water evaporates, leaving behind a crusty mineral buildup.

You may need to use plenty of pipe dope and Teflon tape, and perhaps even some lampwick, to seal the joint while positioning the pipe in the proper direction. After you place the service valves and mixing valve in the proper orientation, you can connect flexible hot and cold water tubing away from the boiler.