Cold, tacky patch in centre of kitchen slab floor

There is a patch, about the size of a cushion, near to the centre of my kitchen floor (concrete) that has darkened and is a little damp/tacky. At first I thought it was glue from when flooring had been put down in the past, but recently (last month or so) it appears to be a little darker. There is an odour of some kind, almost like dirty soapy water in the kitchen. The house is a 1920's terrace and has had a damp proof course in the walls. It is not near to the sink, and there isn't a washing machine in the kitchen. I'm not sure where the supply pipes come into the house and run through the kitchen although the waste pipes go straight out from the back wall and don't appear to have any leaks. Wondering whether there would need to be someone come in and break the concrete to look underneath to check it out, and what kind of cost it may come to. Fortunately, although I've had quite a bit of renovation done, I haven't touched the kitchen yet.