Any plumbers about?

[SuePurblybiltbyElves]

My airing cupboard is in the kitchen and has an odd assortment of pipes coming from an old, recently removed, back boiler (capped off), my hot water tank and two black plastic ones going into the concrete floor.

One of the ones going into the floor has been cut off about an inch above the floor (not sure when, I moved here this year) and is brimming with water. Not gushing out but brims again when I soak some with a flannel. I have no idea where this pipe goes or comes from , I assumed it was an old one that nobody could be bothered to remove from the concrete.

The hot water tank is that only, we have no rads, and is powered by electricity. It appears to be working fine. Any ideas/advice?