[AMc]

First thing to do is a visual check of all the pipes/connections. If there is a leak, you will usually be able to see some pinky/white crystallisation where the coolant dries up. If there is none of that, then it could be an internal leak, so check your exhaust for white steam (this time of year is a bit more difficult, as the air is cooler and you can see the water vapour that occurs from combustion alone...) so this is usually more prevelant after the engine/exhaust has all warmed up.

As above, could simply be due to the temp drop recently causing the coolant to reduce in volume slightly.

Best thing to do is just keep a track of it, measuring it in the same conditions each time. i.e. flat surface/cold engine.

 

[AMc]

Ideally you want to top it up with the correct coolant...water will dilute the mixture and reduce its efficiency and also make it more prone to freezing in the winter. This is extreme, a small top up won't make that much difference, but over time it would.

If you do use water, don't just use tap water...get some de-ionised.