Does this indicate a leak?

I added a third keg to my setup exactly a week ago which is on 10lb co2 gas. To that point my primary regulator reading showed full and the tank had serviced about three kegs prior. So, this new keg was about the sixth or seventh. Last night I saw that the dial had gone to the "change gas" reading. I was surprised because of how abruptly the change happened. There was no change to the settings of each of the three kegs that are connected. I have tried to find leaks but can't seem to locate any. Although I have a connector going through the wall of the fridge that I keep the setup so I can't say for certain that it is not leaking.

Is it unusual to see such a drop when adding a new keg as the co2 becomes absorbed - particularly if the tank may be nearing empty?

The high pressure gauge doesn't tell you how much gas is in the tank, just whether or not there is gas in the tank. The pressure will read pretty much the same (assuming constant temperature) as long as there is liquid gas in the tank. As soon as the liquid gas is gone and you're running on fumes, it will drop suddenly.

The gauge dropping suddenly isn't an idication of a leak, but using nearly 10# of CO2 to carb and serve only six kegs might be. You should be able to carb and serve at least one keg per pound of CO2.

Perform a leak test on your setup. Just disconnect the kegs from the gas system, turn up the second gauge pressure to charge the system to over 20PSI (I have better results at 30-35psi) and then turn off the tank. If the system is leak free, it will maintain the pressure in the lines. If there's a leak, you should see evidence pretty quickly (I usually see it within 30 minutes).

If you determine there is a leak, start shutting off gas to parts of the system. Easiest is often at the manifold. If you're leak free after that, you know it's someplace in the system from there to keg. Turn on one gas feed at a time and check.

Depending on how complex your gas system is, this could take several hours, or a couple of days to hunt down the leak(s) and resolve them all. But, once you've done it, you're pretty much done. Well, until you change a component that is.

I would also pressure test your kegs before filling them. You should also be checking the kegs once filled and purged of air. Being sure the lid O-ring is seated properly, and leak free, is rather important. Although a quick shot of StarSan usually lets you know IF you have a leak there. It's one of the reasons to keep a spray bottle of it on hand.

If you have another regulator, that's not connected to kegs (normally) you can use that to check gas lines as you make them.