I had a similar event on one of my first colony, had a giant building in the center of the map and decided to send my colonists to make that their starting base... (I didn't know they contained mechs) I ignored the warning and a centipede and two scythers proceded to rip my colonists apart.
On my current colony I had two mechanoid buildings, the first one I opened far away from my base with 1 scyther and two centipedes, my colonists fled but the mechanooids stayed guarding that area. They even took out the better portion of three raids and two group of visiting tribals. The second one was right next to my base, I opened it up when I had a massive group of visitors that bravely defended my colonists from the mechanoids until all that was left was a severly damaged centipede. No causulties (for me, the visitors lost about 2/3 of their people). As of now I haven't opened the caskets yet.
On my current colony I had two mechanoid buildings, the first one I opened far away from my base with 1 scyther and two centipedes, my colonists fled but the mechanooids stayed guarding that area. They even took out the better portion of three raids and two group of visiting tribals. The second one was right next to my base, I opened it up when I had a massive group of visitors that bravely defended my colonists from the mechanoids until all that was left was a severly damaged centipede. No causulties (for me, the visitors lost about 2/3 of their people). As of now I haven't opened the caskets yet.