OP states he doesn't have the LBS installed chain.

The number one reason I've seen dozens of chains break is from the outer side plates working off the pin's end. This can happen from poor assembly (if the chain is assembled with a pin that gets pressed into it) and from poor shifting techniques. Yes other reasons happen but these two probably cover 98% of the causes. So when approaching a LBS about whether they miss installed the chain not having the chain in hand makes the question pretty much impossible to figure out. As in the evidence has been erased. Perhaps if you were told about how the first chain broke it might lend some light on the second one. But the OP also states he doesn't have that one either, was it with the bike when the second was installed?

It will be interesting how the LBS handles this with no real evidence. If the Op is a regular customer and has a good relationship with the shop then regardless of the exact why the chain broke (and the extension as to who's responsible) the shop might just eat the cost and install another chain at no cost. If not then it might be a test of how much the shop wants this customer to play nice on social media.

The take away is to keep the broken parts if you want to figure out what happened. Andy