Gundren and his two brothers believed that they found the remains of Wave Echo Cave, and they weren’t being quiet about it. It probably shouldn’t have come as a surprise that Gundren then went missing, but it did. His family is offering a reward for his return, and also…the infamous Wave Echo Cave. I mean…C’MON.
The party makes it to Phandalin with evidence that Gundren has been taken by goblins. Further investigation (…and murder) of the local ruffians reveals that he was supposed to be delivered to someone/thing called the Black Spider.
After making their way into Cragmaw Castle (with a brief dragon-fighting stop along the way), the party has just finished dealing with an ass-load of goblins and is continuing the search for Gundren.