Mystery Molt

My 6 month old Brazilian Black, Teddy, Molted about a month ago. I could never find the reason why, but there looks to be a little piece of old "exoskeleton" on his thorax. Its not huge, but its noticeable.
Hes been doing fine so it doesn't seem to bother him. I was just wondering, is there a way to prevent this from happening with my other T's?
There's also some old molt left on my black widows thorax, its creepy because it still has the eyes. So it looks like a thousand eyes staring at you! It isn't in her way, it's kind of off to the side.
I wasn't around when she molted, and I don't think anyone bothered her seeing as I'm the only one fascinated by spiders here (:
Any way to prevent this kind of stuff?

raising the humidity while they are getting ready for a molt would prevent this from happening, any idea what the humidity is at right now in their enclosures??

Nope. The humidity thingy broke.
The dogs got it while I was changing the cage and tore it up, they destroy everything these days.
Its about 90 something around here in Newnan, and I mist the cage twice a day. Sometimes 3 if it gets close to 100.