I am highly amused at your use of python phrases in regards to mice, like "clutch" and "laid".

Mice can be tricky and like to play head games with you sometimes. Depending on how soon you moved them after you found the first extra baby, the female that was giving birth may not have been finished, and finished having them after they were moved.

Since you haven't seen anymore killed, I would just keep an eye on them. The bits you saw may have been from stillborn ones, and/or the blood might have been from the birthing itself if you happened to catch them at the right time.

I agree with satomi with culling any killers you might find. When I raised rats/mice, if a mother killed her babies, she was automatically a feeder. When I first started out, I had a couple here and there that ended up this way, but after I got rid of those ones, I never had any problems over the next 5 years I raised them.