Did you check out the overflow bottle and all? It's possible your cap is shot and there's a leak related to the overflow container, causing water to leak out through that. Water pumps generally either have a slow drip (I know the Z31 had a "weep hole" to check for leaks) or catastrophically fly apart (see: ex's 95 Jeep Wrangler).
Also, did you investigate while it was cold or hot?