Sounds like it was one of the Gen3's with a three-letter serial number prefix beginning in "E". Some of the frames from that series had rear frame rails susceptible to breaking off, and if it happens Glock will replace the frame free of charge. Even if it hasn't happened, you can call Glock and ask if your serial number falls within that range, and if it does they'll replace the frame.
I've heard some of the earliest "F" serial numbers were also included in that range, though I've never heard of one having the problem.
Curiously enough, I saw a thread with pictures on another forum of a Gen4 G17 frame that had one of the rear frame rails break off. It's the only instance I've heard of that happening outside of the Gen3 E-series.
36k rounds. Wow. When I took my G39 in for inspection, which I do with every used Glock I buy since I live 10 miles from the factory, one of the armorers was talking to another customer about his gun, which had been in several times and they couldn't figure out what was wrong with it. The guy asked about the barrel and armorer said the barrels are warranted for 40k rounds, which I thought was interesting, especially since it's not specified anywhere. Of course, depending on what version of the OM you have, the warranty varies...
In the factory in Austria they have a Gen1 G17 sitting in the lobby with a certificate from Hirtenberger Aktiengesellschaft which states that the G17 sitting next to it was tested for 348,210 rounds with only springs needing to be replaced. I don't know why they stopped at that particular round count.