Sexton beetle.

I had to look this one up, not heard of it before and it made me think of a Stag Beetle at first. It seems they are attracted to carrion. Perhaps there is a deceased mouse around your garden?

Having read all about the creature since it appeared, it is a fascinating character. I have often wondered where all the dead birds, mice etc. go! They take them down to a depth of 60cm which is astonishing and prepare them to feed the young. Ghoulish but efficient!

Mother Nature has some weird and wonderful ways!