1994 1.2 coolant system question

[Cobrac]

I have a 1994 1.2 corsa. Its been driving well for over a year. I have recently lent the car to a family friend who has no car, but she has noticed that its' used some water.

I popped over the other night, and she explained that she thought she had better check the water - so she had a look in the expansion tank and it was nigh on dry, so filled it up. I popped over and had a look and it was near the bottom again. I topped it up with some coolant to the level.

I checked the oil filler cap for mayo, nothing, checked the dipstick, checked for coolant leaks, cant see anything. Let the car tick over for 15 mins or so, temp guage didn't go past 90.

The only thing I noticed was that when filling up the water, it was a browny/rusty colour, now this could be becuase it has been there a long time. But does it signify anything else?

I suppose my best bet would be to drain and refill the system.

Any ideas or advice appreciated.

Thanks