same connection 3 pc one my husband,one my pc,the other one my tenant up stairs

Please I want know How this happend to me, In my husband pc there was some of my history that I was visited days before in my pc,no everything, and he never touched the computer that morning, the night before he left the comuter on,and the next morning I found one window open and the history from that morning,and 100% my husband did not touch his computer that day,after few minute,everything disappeard window and history!!nothing was there anymore,and my husband did not belive me
Please tell how this is possible?
I think the tenant un stairs he doing somenting to us? he is really good with computers.
thank you