Lots of tool rental houses & some electrical supply houses have underground tracers (cable locators) that will find the path of the pipes for you. Often used locators show up on Bonanza or eBay for under 100.00 USD. Try searching for "Wire Locator", "Cable Locator" or "Dynatel".
If the pipes are metal, simply clamp the detector's lead on or around the pipe. If they are plastic, push a metal snake or even electrician's fish tape in to one pipe at a time and clamp to that. Use the rental tracer to find where it travels. That should reveal the pipes' purposes.
If there is no mesh or rebar in the slab, a hobby metal detector will trace the pipes, too.
Are they grey PVC? (Can't tell from photo.) If so, they might be for lawn sprinkler or outdoor lighting. Just Guessing.
Paul