In your question you suggest **talcum or chalk**, and this is what I use under sinks, in cars etc.. 

**Kid's chalk** comes in a variety of colours, which is handy if you need to trace against a white background. You can dust it on or draw it on in horizontal lines.

Sometimes I'll wrap or tape on a piece of **tissue or kitchen towel**, when I don't have the above powders at hand or when it's difficult to blow the powder on with proper aim such as deep under a sink, or all around a piece of pipe.

I also use **soot dust** (from burning some paper or a scented burn stick from my spouse's yoga department) or **graphite** from a dispenser in areas where I need strong contrast against white, of if I have neither of the above at hand. 

For a long lasting mark you could use **drywall setting compound**, which would mark the water trail and then harden for inspection much later. I haven't needed this, but always thought this trick could be of use somewhere. Possibly **cement or grout** could be used too.

The benefit of talcum or chalk, and to some degree soot, is that they can be used in small amounts *without too much worry about toxicity and lung irritation*. Uncured drywall, cement and grout powder are corrosive and the dusting should not be breathed in.