Thanks for the answer. Im pretty sure noone sneaked in, i have multiple layers of walls and doors and the stuff that got lost was deep in my base.
I found a thread were the people have a quite similar problem. All items disappeared
But there it really seems to be a bug. Im not so sure about this at my case. It happened again this morning. I lost lots of steelfire, steel reinforcemnt, isolated wood, … And again only the valuable stuff in the benches got lost. The tar and hide in the tannery is gone, yet the leather is still here… And again nothing in the Eventlog.