I've seen this happen with all the utility bots. I kept wondering where my bots were getting off too when they kept disappearing until I finally saw it happen with a party bot. It appears that if you don't repair a damaged bot within a certain amount of time, it will self destruct into a pile of rubble. While I don't mind it happing too much, it does appear to happen too quickly in my opinion. It is somewhat annoying that after you've spent all that time and effort building and upgrading the things that it would self destruct in less than a day after it broke down.
