I was on my survival world (single player), putting some items into a chest until I heard the tripwire. I checked around my house & there was no mobs. I watched the tripwire for a while & saw both ends disconnect from the string, then reconnected.

Edit: It happened again. I managed to get a screenshot of it before it went back to normal. (I'm using Sphax PureBDcraft 128x Resource Pack). Sorry I got one side of tripwire, I'll try & get both sides when it occurs again. Sorry, I don't know how to reproduce this.