If you could still see the coolant in the tank, then the chances are that it's not taken on any air. Top up, and as a precaution take a big tub of pre mixed coolant with you (5 litre tub i'd suggest) on your trip. You'll be glad you did if you have to top up again in the middle of nowhere.
The drop in coolant does need to be investigated, it could be a tiny weep, or a big leak, who can tell without further investigation? The tiny weep, (if that's what it is) may well not remain a tiny weep with use, and could turn into a huge leak, which may well catch you out.






